Mike Pillsbury, a teenage boy who believes that he is an alien and does not belong on Earth, uses a satellite dish one night to send a message to outer space, requesting to be rescued. Two weeks earlier, Mike is telling a fictional story to his friend Nick, and Nick's younger brother, Jay. In Mike's story, a war breaks out between two different alien groups, resulting in a baby alien being sent to Earth to be raised as a human. Nick becomes annoyed with the story, as it represents Mike's unusual feelings that he is not meant to be on Earth. Mike is injured during a football game and sees an alien as part of a hallucination. Mike then wonders if football is the right sport for him, but his father, a former football player, insists he stick with the sport.
Mike is surprised when Katelyn Sandman, a popular girl at school, requests his help in decorating the school for an upcoming Halloween dance party. Mike's foe, Scott Schriebner, who is a player on Mike's football team and a friend of Katelyn, sabotages the light and music equipment during the Halloween party. Mike, upset about the party, abruptly leaves and sends his message to outer space later that night. A thunder strike subsequently destroys the satellite dish. The next day, an alien dog named Barnabus appears, with the ability to speak through a translating device. Barnabus explains that because of Mike, the stargate door has been opened. Barnabus tells Mike that he represents a galactarian organization specializing in disaster relief and rights violations, and that he has arrived to answer Mike's plea to be taken off the planet. Barnabus explains that time is limited, as other aliens with ulterior motives are travelling through the stargate to find Mike.
Mike, who believes he is hallucinating, later meets The Bom, an alien lawyer insistent on having Mike sue Earth for millions of galactic credits on charges of suffering the planet's substandard living conditions. During a telephone conversation with Katelyn, Mike is disgusted by The Bom's way of consuming food, leading Katelyn to believe that Mike is distracted and not listening to her. The Bom angrily leaves after Mike refuses to sue. Nick believes that Mike is going crazy after being told about the aliens.
When Barnabus is informed of The Bom's visit, he tells Mike that he has opened a can of worms by sending out his rescue message, as it signaled to the universe that Earth has advanced to a certain technological level. Because of this, Barnabus says Earth is no longer eligible for intergalactic protection for primitive life, even though Mike blames the message on the thunder strike and does not understand how he actually contacted alien life. Mike accepts Barnabus' offer to take him to a place where he can be alone, but first tries to make amends with Katelyn. However, The Loafer Alien arrives and prevents Mike from meeting with Katelyn, who believes he has rudely ignored her after insisting that they talk. The alien proposes becoming Mike's agent, representing him throughout the galaxy and eventually having him star in a weekly television series about his life as an immigrant. Mike declines the offer.
Nick is stunned when multiple aliens arrive with various offers for Mike. That night, Katelyn visits Mike's house and also becomes aware of the aliens. Mike prepares to make a deal with one of the aliens to be taken off the planet, but the aliens retreat as a Thoad – a dangerous alien that enjoys capturing fine specimens – approaches. Jay unexpectedly arrives and is sucked into the stargate by the Thoad, and taken to the alien's home planet. Barnabus says the Thoad will continue capturing specimens until he finds the perfect one.
The children ask Scott to act as bait for the Thoad in order to get through the stargate leading to the Thoad planet, where the Thoad is safe from intergalactic authorities. Scott is initially skeptical about the children's alien claims, but they convince him that he is the perfect specimen for the job. The stargate opens to pull Scott in, briefly allowing Barnabus and the other children to go through it as well. Barnabus and the three children end up in a cave where the Thoad keeps his zoo of captured species from other planets. Among the specimens is an alien identical to the ones Mike described in his stories, although he is not sure how he could have knowledge of such a creature. The Thoad, in his human form, confronts Barnabus and the children.
The Thoad transforms into his frog-like alien form, but is then temporarily contained within a cage. Mike uses the Thoad's key to release the captured specimens, including Scott and Jay. Barnabus and the children return to Earth, but the stargate stays open long enough for the Thoad to follow them. Barnabus contacts the intergalactic police to have the Thoad arrested. At a football game, Barnabus informs Mike that his transmission has been deemed accidental and that Earth is classified again as a protected planet.Michael Shulman – Mike Pillsbury
Erika Christensen – Katelyn
Adam Wylie – Nick
Andrew Ducote – Jay
Garrett M. Brown – Mike's father (uncredited)
Lee Garlington – Dana Pillsbury, Mike's mother.
Brighton Hertford – Jill Pillsbury, Mike's sister.
Marcus Turner – Scott Schriebner
Chris Davies – Ryan
Marie Stillin – Mrs. Nickerson, the teacher in Mike's computer class.
Jessica Murdoch – Katelyn's Friend
Terry David Mulligan – Coach Trembly
Hrogather Matthews – Thoad's Human Form
Brian Steele – Thoad's Creature Form
Malcolm McDowell – Barnabus
Bruce Lanoil – The Bom
Wally Wingert – The Loafer Alien
David Coburn – The Jarm, an alien that wants Mike to market his "Jarmonica" slicing utensil.
Tara Strong – Lula, an alien that wants to date Mike.
Peter Kelamis – Intergalactic Cop
J.D. Hall – Thoad's Creature Form
After her first rewrite on the film's script, Kathy Mackel was replaced by a new writer who did three rewrites. However, Disney was not impressed with the script and rehired Mackel, although very few of her suggestions were used for the script. Mackel subsequently contested for sole screenwriting credit on the script and won through the Writers Guild arbitration process.
The film was shot in Vancouver, British Columbia. Prosthetic and animatronic effects were used for the film, and were provided by Steve Johnson's XFX Group and Stargate Films, Inc.
In December 2015, Megan Daley of Entertainment Weekly ranked Can of Worms at number 25 on a list of the top 30 Disney Channel Original Movies. Daley wrote, "While Can of Worms isn't as iconic of a PG-fright fest as Halloweentown, it does have a Malcolm McDowell-voiced pup — and an alien has never been so friendly and adorable. (Bonus points for a special appearance by a pre-Parenthood Erika Christensen.)"
In May 2016, Aubrey Page of Collider.com reviewed each Disney Channel Original Movie released up to that point and placed Can of Worms at number 38. Page praised the film's "surprisingly creepy alien creatures" and noted the involvement of Paul Schneider, Mark Mothersbaugh and Malcolm McDowell, writing, "There's a lot of talent at work here in Can of Worms. [...] But that's not to say this one is good – it isn't. In fact, I certainly wouldn't have remembered this one at all if [the film's] wildly creative creatures hadn't kept me awake for a long, sleepless night in 1999."
Although not shown for many years, the film was broadcast as part of Disney Channel's 2006 "Hauntober Fest" in October. On October 23, 2007, the film reappeared as part of the Halloween film line-up on the Disney Channel. The film was a part of "Wiztober" in 2008 and 2009, which is a mix of Halloween films. Also, the film was re-aired on Disney Channel on October 9, 2011 as part of Disney's "Monstober" event, and on October 6, 2015. The film was also aired in May 2016, as part of Disney Channel's celebration of 100 Disney Channel Original Movies.