It is Labor Day weekend, 1946. Young, virginal Ida Parsons innocuously plays as her father hosts a raucous party. Amid the festivities, an older, drunken man named Tom Rice staggers outside and propositions Ida. When she refuses, he chases her into the woods and brutally rapes her; her dogs break out of their pen and they attack and fatally maul Ida's rapist.
The film flashes forward to 1982. Preppy brothers Eric and Nick are borrowing their father's yacht to take their girlfriends, Sandy and Donna, on a weekend outing along with their sister, Carla. At the outset of the trip. As the tensions rise between Nick and Eric, Donna and Sandy engage in girl-talk, and geeky Carla silently laments that she is the sole member of the cruise who came along without a significant other.
That night, fog settles in; Eric and Nick, hearing cries out on the water, discover and rescue a shipwrecked fisherman named Bert. Bert informs them that he wrecked offshore Dog Island, the home of lumber baroness Ida Parsons, who has used her family fortune to hole herself up on the island for the past thirty-five years; only making two annual voyages onto the mainland for necessary supplies, and never speaking to anyone during these trips. Recovering from the onset of hypothermia, Bert tells the quintet a campfire story about the savagery of the wild dogs which roam Ida's island, acting as her sentries. Wrecking their boat after Nick, in a panic, attempts to steer the boat back to the mainland.
Donna, Eric, Sandy, Nick, and Bert wash up on Dog Island; Bert has been seriously wounded, and Carla is nowhere to be found. Nick wanders off into the woods, and is subsequently killed by a hulking figure that breaks down the shed where Nick hides. The next morning, Sandy and Eric go off onto the island, hoping that Ida Parsons will help them get back to the mainland. Shortly after they leave the beach, Bert goes into shock, and Donna desperately tries to warm him by stripping to the waist and lying topless across his shirtless torso. Seconds later, the same figure which killed Nick sneaks up behind Donna and Bert, fatally hurling Donna against a rock wall and decapitating Bert.
At the center of the island, Sandy and Eric discover Ida's fortified cabin, as well as the fact that all of Ida's dogs have died long ago, their mutilated skeletons lying in their pens. In Ida's boathouse, the duo discover Carla alive hiding under a tarp; she apparently washed up at another point on the beach and made it to the compound in the middle of the night.
In the course of exploring Ida's compound, Eric, Sandy and Carla discover a dust-covered nursery full of antique toys, and a cobweb-covered crib; they also discover Ida Parson's diary, which contains insane, rambling passages about giving birth to a sick child, which she intends to keep sinless by secluding him from all the evils of the outside world. As they continue exploring the house, Sandy comes across Ida's skeletal corpse, positioned in repose in her bedroom. The group decides to collect supplies and head back to the shore to collect the rest of their party so they can formulate an escape from the island in Ida's old rowboat. While exploring the basement, they discover the corpses of Nick and Donna and flee to the beach in a panic.
Eric and Sandy deduce that Ida Parson's son was the one behind the murders, left insane by his life of solitude under the care of the imbalanced Ida; with nothing to do but learn from Ida and explore the wilderness, he's become immensely strong, a capable tracker and hunter, and is thoroughly convinced that all outsiders are a threat to him and his mother. With the death of Ida, he was left without any basis for reality, and ended up eating the dogs to survive.
Eric and Sandy go back to the house and get the matches that Sandy dropped earlier. Ida's son attacks, breaking down the door. Eric attempts to fight using a broken branch, but the mutant grapples with him and fatally breaks Eric's back; once it has killed Eric, it turns it attention to Sandy and chases her upstairs into his mother's bedroom, where Sandy wraps a blanket around her head and, playing Ida, convinces the mutant to leave his mother's bedroom. But when Sandy leaves the room, the mutant is there and pursues Sandy out of the house and to the boathouse. Sandy runs headlong into Carla at the boathouse and the mutant, close behind, grabs Carla and crushes her face and kills her. Sandy manages to lure the man into Ida's boathouse, which she sets on fire; Ida's son is severely burned in the blaze, but still manages to leap from the water and attack Sandy, chasing her up a hill where she yanks a sharp signpost from the ground and impales the mutant with it. As he dies, we finally see his burned and deformed face.
Traumatized by the death of her friends and the murder she has been forced to commit, Sandy sits alone on Ida's dock, strongly resembling a scarred and traumatized Ida.Janet Julian as Sandy Ralston
David Wallace as Eric Simmons
John Wildman as Nick Simmons
Janit Baldwin as Carla Simmons
Joy Boushel as Donna Blake
Layne Coleman as Bert Defoe (as Lane Coleman)
Shay Garner as Ida Parsons (as Shea Garner)
Page Fletcher as Tom Rice
John McFadyen as Ed Parsons
Garry Robbins as Ida's son
Critical reception for the film has been mixed to negative. AllMovie's review of the film was negative, calling it a "plodding horror bore" and commenting, "whatever interest this deathly dull flick may have mustered is completely obscured by some of the murkiest cinematography on record; the fact that nearly every scene is shrouded in complete darkness may prove a blessing in disguise."
Tim Brayton from Antagony & Ecstasy gave the film a negative review calling the film "Junk" and criticized the film's characterization as being "of limited interest" but complimented the film's unique villain and being better acted than most slasher films. However the film has received some positive reviews, Kurt Dahlke from DVD Talk gave the film a positive review stating "Though derivative of other Slasher Movies, plagued by the annoying improbabilities of all A-to-B horror efforts, filled with bickering dorks, and pretty slow to get started, Humongous eventually gets to a weird, frenetic payoff". Tom Becker from DVD Verdict also gave the film a positive review. In his review on the film Becker commented, "While it might not be the shocker it should be, Humongous is a pretty entertaining bit of horror fluff from the days when people took this stuff seriously". Jack Sommersby from eFilmCritic.com awarded the film a score of 3 / 4 stating, "It's perfect for audiences looking for an undemanding, scary good time, along with those longing to see the drop-dead gorgeous Janet Julian in a skimpy bikini". Author and film critic Leonard Maltin gave the film one and a half out of a possible four stars, calling the film "slow-moving", and criticizing the film's overuse of cliches, lack of visibility, and lack of clear shots of the monster. The film currently has a 0% "Rotten" rating on Rotten Tomatoes with an average rating of 3/10 based on five reviews.
Humongous made its debut on DVD on Nov 15, 2011 where it was released by Scorpion Releasing.