Monster High is enjoying high spirits during a swim meet as mermaid/ghost hybrid Sirena Von Boo and Australian ghoul Lagoona Blue earn very high scores from the judges, including Headless Headmistress Bloodgood, Mr. Where, and Mr. Hackington. In the crowd, Spectra Vondergeist is struggling to find any new piece of gossip to put on her blog since she has posted nothing new in a while. Angered over her recent suspension from the Fear Squad due to plagiarism on an essay, Toralei Stripe tells Spectra a lie about Clawd, Clawdeen, Howleen, and the other werewolves having were-fleas. As soon as Spectra posts this little tidbit, and the students read it immediately, many of the students around Howleen put distance between her and themselves, Heath Burns and Deuce apologize and take their leave from Clawd, and Clawdeen, furious about that outright lie, causes the Fear Squad pyramid to collapse into the pool, much to Toraleis amusement, and cause the squad to be accidentally electrocuted by Frankie Stein.
Afterwards, Clawd, still believing the gossip, keeps his distance from Draculaura, having other guys like Johnny deliver gifts to her. Afterwards, Draculaura notices that she seems to have picked up a ghost haunting her as her locker acts up under its own power, she speaks out of turn during Mr. Rotters class, and the ghost fixes up her hair. Concerned, she goes to speak to Sirena, Twyla, and Rochelle Goyle. Deciding to speak to Spectra about it, they find her as Clawdeen chews her out about her were-fleas post from earlier. When Draculaura mentions being haunted by a ghost, Spectra suddenly becomes deeply concerned, and leaves without explaining herself. Still wanting to help Spectra with writing honest, truthful pieces in her blog, Clawdeen goes after her, followed by Dracluara, Twyla, Rochelle, and Sirena. When they find her in the attic of the school, they watch as she takes a key and opens a mysterious doorway and disappears into it, after nearly discovering them when they made some noise. Still keeping to her promise to help Spectra, Clawdeen goes after her, and the others follow. They soon find themselves leaving Monster High aboard a Viking-esque ship, piloted by the daughter of the Grim Reaper, River Styxx. Her father is away at a convention in Las Plagus, so she has the boat all to herself. Thats when Spectra appears, concerned as to why they are on Rivers boat. Its then that she and River explain about the Ghost World, and how many of Spectras old friends still live there while many, like her, Operetta, and others went to Monster High instead. Rivers boat is going to take them back to Spectras old school, Haunted High.
Its at that point that River asks Spectra why shes coming home to the Ghost World, and Spectra explains that Draculaura is being haunted. River finds that impossible, until she remembers the legend of the Red Lady, the most scariest, and notorious, ghost in the Monster World, who was believed to have found a way to quickly travel between the Ghost and Monster Worlds, building her infamous reputation with every ghostly crime. She was finally apprehended and locked away with it believed that her crimes were so heinous, it would take centuries for her to work off her debt to the afterlife.
Soon after River finishes telling the story, the ship falls under attack by a ghostly pirate ship and its crew, led by Vandala Doubloons and her pet, Aye, who was only doing it for a Ghost Pirate Class project and needed River to sign off on it. Vandala panics upon seeing Draculaura and the others, known in the Ghost World as "solids," and worries about their presence at Haunted High. River tries to help by disguising them in Grim Reaper robes, but Sirena does not need one due to her being half-ghost herself. Also, due to Drauclaura, Clawdeen, Twyla, and Rochelle being solid, Spectra has to help them enter Haunted High by opening the doors for them. Sirena is amazed that her parents never told her about Haunted High, while Spectra is amazed by how its like nothing has changed, until River points out the chains that many ghosts are dragging after themselves.
Those are Detention Chains, handed out to the ghosts by Haunted High headmistress, Principal Revenant. The chains are handed out to rule-breakers with the job of working them off being extra homework, or chores. Vandala remembers how long it took to work off her last set of Detention Chains, and worse still, youre restrained from ever leaving Haunted High until you work the chains off. As the ghosts and solids head to the Creepateria to ask about Draculauras haunting, none of them notice Revenant watching them with suspicion from her office window...
In the Creepateria, its not long before the ghouls spot Spectras old classmate, Kiyomi Haunterly, who apparently has gone from the most shy ghost at Haunted High, to the most popular ghost. However, when introduced to Spectras friends, Draculaura seems to have some familiarity with Kiyomi, wondering if theyve met before, before Kiyomi is forced to leave for some reason. Brushing it aside, Spectra begins asking around about Draculauras haunting, before attention is drawn to Porter "Paintergeist" Geiss spray-painting graffiti regarding the Detention Chains on the wall, encouraging the other students with wanting freedom from their Detention Chains. Unfortunately, this attracts the attention of Revenants Hall Moanitors, ghosts of hauntings past, present, and future, who begin apprehending any ghosts who are breaking the rules, will break the rules, or are repeat offenders. When they catch Porter doing his spray-painting again, they prepare to shackle him, but he attempts to flee, which attracts Spectra with having found her first scoop. After a lengthy chase through Haunted High, Porter crashes into Spectra, leading to him finally being apprehended by the Hall Moanitors, as Revenant herself arrives to ensure he is punished for his actions.
Unfortunately, when Spectra speaks up about Porter crashing into her, Revenant wonders who she is. When Spectra mentions shes visiting from Monster High, Revenant grows suspicious of her friends, and removes their hoods, exposing them in front of everyone there as solids. Revenant immediately asks for punishment on Spectra, but when the Hall Moanitor of the Past says no rule in Haunted Highs rulebook about bringing non-ghosts to the school has been written, Revenant has the Hall Moanitor of the Present add it in, much to Spectras annoyance, before Revenant leaves for other matters.
Porter is able to create a distraction to allow Spectra and the others to flee on Rivers boat, but the Hall Moanitors give pursuit, only to discover that the Detention Chains have no effect on solids like Clawdeen, Draculaura, Twyla, and Rochelle. Although they make it back to Monster High safely, one Detention Chain follows them through and captures Spectra when they let their guard down, dragging her back into the Ghost World to imprison in Haunted High, despite their attempts to save her. With Revenants new rule about no non-ghosts allowed in effect, the only option is to turn the ghouls into ghosts so they can return to Haunted High and get Spectra back. Luckily, Twyla knows just how to do it.
Back at Haunted High, Spectra is brought into Revenants office, where Revenant confronts her about Monster High and the other Ghost World residents who are enrolled there. Despite Spectras assurances that all monsters are welcome at Monster High, Revenant still punishes her with working off an assignment of 10,000 essays, then doing it again. When Spectra worries about her blog, Revenant shows little sympathy for her plight. Before she goes to suffer her fate, Spectra asks about Draculauras haunting, but Revenant fiercely denies it, even showing some nervousness about there not being another haunting in the Monster World before Spectra brings up the Red Lady legend, which Revenant agrees with before Spectra departs to her fate. However, after Spectra leaves, Revenant, interested in the number of students from the Ghost World enrolled in Monster High, begins concocting a plan to send her Hall Moanitors into the Monster World to increase Haunted Highs attendance...
As for Spectra, she does her best to work on blog posts while having a good talk with Porter, who is puzzled by how Spectra writes for fun, but they soon seem to develop a friendly relationship before Spectra leaves him to his own interests.
Back at the Monster World, Twyla brings the others to the Boogey Mansion to show them her fathers Boogey Sand that he uses for his work. While they look for the bottle that will turn the ghouls, sans Sirena, into ghosts, back in the Ghost World, the Hall Moanitors begin their assault on Monster High to drag as many of the Ghost World students back to Haunted High, wrapped in Detention Chains. Twyla finally finds the right bottle, and after Clawdeens werewolf allergies sneezes the sand over them, Clawdeen, Draculaura, Twyla, and Rochelle, have been turned into ghosts, just until they use the sand that will revert them back to their original forms. Sirena is ready to head back to Haunted High at once, but the others still need time to adapt to their new, ghostly forms.
Back at Haunted High, Porter and Spectra continue to build their relationship as they find themselves unable to leave while bound by their Detention Chains, and Porter requests Spectras help with a feeling he has about Revenant and her being up to something sinister, especially with all the rules and detentions. Meanwhile, the Hall Moanitors Detention Chains make their way into Monster High and begin to round up all the Ghost World students, including Operetta, Johnny Spirit, Scarah Screams, and others. Back at Haunted High, Porter and Spectra go to the library to look for information on Revenants past, while Sirena and the others arrive, and quickly are spotted by Kiyomi and Vandala when they notice the new ghosts struggling with their new forms. While Kiyomi and Vandala help teach Clawdeen, Draculaura, Twyla, and Rochelle how to control their ghost forms, Porter and Spectra come up empty on any information regarding Revenant. As they continue to ponder over how if there is no information on Revenant, that proves shes up to something, Spectra is horrified when she sees many of her classmates from Monster High, including Operetta, Johnny, and Scarah, being welcomed into Haunted High by Revenant. Spectra comes down to see them after Revenants welcome speech, and promises to help get them home to Monster High. Back with the others, Vandala and Kiyomi finish with training them, and they head off to find Spectra.
As for Spectra, after overhearing some gossip from another ghost and yelling at him about it, Spectra realizes exactly what Clawdeen meant by how gossip can be hurtful, and vows to end posting gossip on her blog and only post truthful news. She is soon reunited with the others, and she tells them about what happened to Operetta, Johnny, Scarah, and the other ghost students from Monster High. As they watch as the Hall Moanitors contiune to persecute ghosts for more silly and pointless rules, much to their own amusement, the Hall Moanitors catch Kiyomi and threaten to punish her for another silly rule, when Draculaura and the others try to protect her. This results in the Moanitors summoning Revenant, who upon seeing how the solids became ghosts, demands to know how they did it. When Porter tries to intervene, Revenant grants the Moanitors permission to shackle Sirena, Twyla, Rochelle, Draculaura, and Clawdeen, in Detention Chains as well. Kiyomi sacrifices herself to protect Dracualura from the first set from the Hall Moanitor of the Present, and tells them to run, with Porter trying to help them out as the Moanitors give pursuit. Porter is able to lead the ghouls into the safety of a secret room, where when Spectra and the others see the amazing artwork inside that he did on his own, Spectra is amazed by it, unable to comprehend that Porter could paint something so beautiful. They also find some antique furniture that he found when he first came across the room, and when the mention of the Boogey Sand is brought up, Draculaura realizes that Kiyomi is the ghost haunting her. Kiyomi confesses and reveals how she was so shy, but upon seeing Monster High and the students there with their freaky flaws, it helped her to overcome her shyness and why she is so popular now at Haunted High. Now she feels remorseful over how it was her fault Spectra was given Detention Chains. Draculaura and the others assure her that its not her fault, its Revenants, and Spectra decides to go talk to her. Porter asks her not to do it, fearful of losing her, but Spectra assures him shell be fine, and leaves. Porter goes after her after noticing the looks hes getting from the others due to how he has warmed up to Spectra in the time shes been imprisoned at Haunted High.
Spectra eavesdrops on Revenant as she voices her frustration at how if she could know the secret to turning monsters into ghosts, she could double Haunted Highs attendance. The Hall Moanitor of the Present worries how the other students would react, but Revenant assures him she has ways of dealing with that. As Spectra writes everything down, shes soon discovered by the Hall Moanitor of the Future, and brought in to Revenants office. At that point, Revenant reveals herself to have dozens of Detention Chains attached to her, having been passing them onto the students to work off so she does not have to. Luckily for Spectra, before Revenant can pass anymore of her chains onto her, Porter is able to provide a distraction to give Spectra time to escape, before he is personally wrapped in more Detention Chains by Revenant herself. Revenant offers Porter a deal: Tell her where the Monster High students are hiding, and she might undo some of those Detention Chains on him. Only after she tightens them a bit does Porter agree to work out a compromise...
Back in Porters hiding place, Kiyomi reveals how she could travel to Monster High to see certain things, similar to the Red Lady, and haunt monsters as well. However, theres a catch: Kiyomi cant risk revealing the Ghost World to the Monster World as many ghosts fear the monsters are out to get them. Although Draculaura assures her it will take some time, thats one thing they dont have as Spectra returns, out of breath, but before she can explain everything, Revenant and the Hall Moanitors find them, and shackle everyone there in Detention Chains. Revenant reveals that Porter sold them out, and proves it as she brings him in to reveal all his chains are gone, further breaking Spectras spirit as the Hall Moanitors cackle at her to further taunt her. Porter tries to apologize, but Spectra will have none of it.
Revenant then puts Haunted High on lockdown, and spills the beans to the students that monsters have invaded the school, further spreading paranoia to the student body, and then declaring her intentions to ensure that the monsters pay for their threats against the Ghost World. As for Draculaura and the others, Revenant offers a deal to them as well: Give up the secret to what turned them into ghosts, and shell reduce their Detention Chains. The ghouls refuse, but Revenant will get what she wants, one way or another. Thats when she discovers Clawdeen and Spectras intentions to publicize the truth on her, and confiscates Clawdeens notebook. From that, she discovers the Boogey Sand, and sends the Hall Moanitors to acquire it, confiscating Rivers fathers skiff as well to travel to the Monster World.
With the fate of both of the Ghost and Monster Worlds now sealed with Revenants flunkies on their way to acquire the Boogey Sand, Spectra knows who the full blame for this hopeless situation falls on: Porter "Paintergeist" Geiss.
The ghosts go to confront Porter, and as he tries to explain himself, the ghosts will have none of it, with Sirena even having Vandalas pet, Aye, attack him. Hes finally able to reveal that he stole the key Revenant uses on the Detention Chains, and transfers all of the ghouls chains to himself as punishment for his actions. This convinces the other ghouls to forgive him immediately. Shocked by this show of sacrifice, Spectra is uncertain of what to say, but Porter entrusts her with the key to help free Operetta and the others, and stop Revenant for good. He even offers her the old furniture he found, revealing he restored it to working order as another gift to Spectra to show how sorry he is.
Meanwhile, at the Boogey Mansion, the Hall Moanitors go through several of the Boogey Sands before finding the ghost one and contact Revenant to let her know theyve found it. As she assigns them one more job, back at Haunted High, Spectra and the others get to work transferring all of the Detention Chains from their fellow Monster High students onto Porter to hold onto while ensuring that Revenant never finds out about the key being missing. As more and more Detention Chains are put on Porter, he can only look back with pride at his art piece of freedom from the shackles of the Detention Chains.
With all the Monster High students free, Sirena leads them to Vandalas ship for transport back to Mosnter High, much to everyones celebration. Before they leave, Spectra goes to say thanks to Porter one more time, but upon seeing all the Detention Chains weighing him down, although worried about him, she and Porter exchange a few more words before they hear Revenant make an announcement about a gathering in the main lobby of the school. Deeply worried by what Revenant means, Spectra goes to investigate, despite Porters pleas to leave while she still can. When Spectra arrives, Revenant speaks to the gathered Haunted High students, concealing the truth about the theft of the Boogey Sand as being of her own creation, and using it on the Monster High students to turn them into ghosts, and thus give them their well-deserved punishment for making the Haunted High students live in fear of them. While the Haunted High students cheer at this blatant lie of revenge against the Monster World, Spectra, fed up with all the lies and gossip, heads off to stop the Hall Moanitors, some way, some how. However, she returns to get some last minute time with Porter before she heads off.
With time running out, as Vandala heads for Monster High, Spectra reveals Revenants plan for the Monster High student body, then realizes herself that Revenant is the Red Lady of legend. The chains she has been putting on the Haunted High students were the ones she was given when punished for her crimes centuries ago. While everyone else heads for Monster High to stop the Hall Moanitors, Spectra returns to the Haunted High library and finds the book with the information she was looking for. As Spectra and Porter go after Revenant, the others put their hopes in Kiyomi to open a window to get them to Monster High before the Hall Moanitors do. Kiyomi succeeds, and the ghouls return to Monster High well ahead of the Hall Moanitors, who arrive in Monster Highs main lobby, causing panic amongst the students there, including Manny Taur, Deuce Gorgon, Iris Clops, and others.
Vandala and the others arrive just as the Hall Moanitors open fire with Rivers skiffs confetti cannon, with Manny and Iris being the first victims to be turned into ghosts and shackled with Detention Chains. Before they can fire on Lagoona and Gil and make them ghosts and shackle them with Detention Chains as well, Vandala grabs their attention and steals the Boogey Sand, forcing the Hall Moanitors to go after them to retrieve it. As Revenant furiously asks what else could go wrong, her worst fears are answered as Spectra reveals Revenants true identity as being the legendary Red Lady to all of Haunted High. As Haunted Highs students are in disbelief at learning the truth, Revenant appears, retrieves the key to her chains, and reveals her true identity, much to the shock of the students. Her plans in ruins, Revenant decides to give Spectra all of her chains as punishment for Spectra preventing her from regaining her freedom.
Back at Monster High, the other ghouls try to outrun the Hall Moanitors, but theyre too fast in Rivers skiff. Kiyomi has had enough, and prepares to open a window to send the Hall Moanitors back to the Ghost World. Seeing she cant do it alone, the other ghouls lend their ghost strength to Kiyomi, and succeed at opening the window wide enough to let the skiff through, much to the shock of the Hall Moanitors.
Back in Haunted High, Revenant prepares to transfer all of her chains to Spectra, but as she does, Rivers skiff emerges in the path of the chains, and Revenant only ends up tying up her own flunkies along with Rivers skiff. Although not what she intended on, Revenant is still free of her own shackles, and prepares to return to the Monster World to resume her crimes. Her victorious cackles are soon silenced when she finds the window refuses to allow her through. Revenant cant believe it, and worse, she finds that she is being re-shackled in more chains. She thinks that someone else is doing it to her, but Spectra reveals that it is Revenants own fault. Every wrongful detention she gave out, every unjust action, only served to further punish the Red Lady with more and more chains. Revenant is on the verge of tears at being unable to comprehend how her own actions have led to her inevitable downfall, but Spectra makes it clear that gossip can only led to bad things, and deep down, Revenant herself knew that what she was doing was wrong.
As Spectra makes a solemn vow to end spreading gossip for good, Revenant also sees the error of her ways, and releases the Haunted High students of their Detention Chains, taking them all back, including the ones she wrapped around Porter, Rivers skiff, and the Hall Moanitors. Afterwards, she asks for the students forgiveness and allowing her to remain as their principal as she works off her chains she brought upon herself for her own mistakes. She dispels the key to her chains, and watches as this first act of repentance causes the first of her chains to vanish due to her good deed, but Revenant knows it will take a long time for her to work off all of them and earn her freedom, but at least she is on the right path now.
As the students cheer Revenant on, showing that they forgive her and are with her on her road to redemption, Porter congratulates Spectra on her heroism. Soon after, when some of the ghost students ask about the gossip on Monster High being wrong, Spectra decides its time for Monster High and Haunted High to get to know each other much better than before.
Soon after, both schools are enjoying the company of one another. Revenant does whatever good deeds she can to work off her chains, such as serving drinks to the guests, Vandala helps Frankie, Cleo, and Ghoulia attempt to sound like pirates while Toralei and her flunkies get their comeuppance when Vandala reveals theyve got sea-fleas. The Hall Moanitors also attempt to work off their debts to society as they swab the deck of Rivers skiff, and River works with Holt manning the DJ station. Soon, Clawdeen and the others are returned to their normal selves, and the party continues as Spectra and Porter add a little graffiti of their newfound, and romantic, relationship on the wall behind them.
In a post-credits scene, the Hall Moanitors return to the Boogey Mansion to have some fun with the Boogey Sand after earlier events with looking for the ghost variety, but when caught by Twyla, she lets them have their fun.Laura Bailey - Lagoona BlueCam Clarke - Heath Burns, Hoodude Voodoo, Mr Rotter, Ghost StudentDebi Derryberry - DraculauraErin Fitzgerald - Spectra Vondergeist, Rochelle GoyleJonquil Goode - TwylaMarcus Griffin - Clawd WolfTodd Haberkorn - Porter Paintgeist GeistKate Higgins - Frankie SteinJoy Lerner - Kiyomi Haunterly, Past Hall MoaniterAudu Paden - Ghoulia Yelps, Manny Taur, Present Hall Moaniter, Future Hall Moaniter, Cuttlefish AyeAshley Peterson - River StyxxPaula Rhodes - Sirena Von Boo, Iris Clops, Scarah ScreamsCindy Robinson - OperettaSalli Saffioti - Clawdeen Wolf, Cleo de NileEvan Smith - Deuce Gorgon, Johnny Spirit, Boy Ghost, AnnouncerHaviland Stillwell - Vandala Dubloons, Avia Trotter, Girl GhostGeeg Friedman - Bontia FemurKaren Strassman - Principal RevenantJosey Montana McCoy as Neightan RotAmerica Young - Toralei Stripe