A young boy named Harley is an ardent admirer of The Banana Splits. On his birthday, he attends a live taping of the show at Taft Studios, accompanied by his parents, Beth and Mitch, as well as his older half-brother Austin. Excited, Harley waves at the show's characters — Fleegle, Bingo, Drooper, and Snorky — as they cruise in one of the Banana Buggies, narrowly avoiding an accident. However, the festive atmosphere takes a dark turn when Andy, the Vice President of programming, discloses to producer Rebecca that the show is slated for cancellation after the taping.
Following the recording, the characters, who are revealed to be robots, receive the grim news of their show's termination from their colleague Stevie. In a shocking twist, they embark on a spree of violence throughout the studio, starting with Drooper brutally forcing a lollipop prop down Stevie's throat. Outside, Beth discovers that Mitch has been unfaithful, citing his feelings of neglect as she dedicated her time to Austin and Harley. As tensions rise, Snorky, driving the Banana Buggie, tragically runs over Mitch as he attempts to leave.
Meanwhile, a couple, Thadd and Poppy, joyously engaged, find themselves in Fleegle's Magic Shop set. Their happiness is short-lived when Fleegle viciously saws Thadd in half, and as he prepares to do the same to Poppy, he is interrupted by Harley and his friend Zoe searching for Snorky. Fleegle captures the children along with another girl named Parker in the workshop where the characters were constructed.
As chaos unfolds, Beth, Austin, Rebecca, and show host Paige begin to piece together the horrific events when Parker's father, Jonathan, arrives severely injured, adding urgency to their search for Harley and Zoe. They encounter Poppy, who is devastated by Thadd's fate, and persuade her to join their desperate quest.
Inside the studio, Harley, Zoe, and Parker meet Karl, the creator of the Banana Splits, and plead for their release. Just as hope seems within reach, chaos ensues with the arrival of Drooper, dragging Bingo, injured from an earlier attack by Beth. In the confusion, the kids manage to outsmart Karl, trapping him instead.
Meanwhile, Rebecca and Jonathan find themselves at the mercy of Fleegle and Drooper, who force them to navigate a twisted game called "The Sloppy Time Course" for their freedom. The horrific events culminate with Fleegle stabbing Jonathan and Drooper grievously injuring Rebecca.
Back on the hunt, the group — Beth, Austin, and Poppy — confront Karl for answers, but he offers none, only leading them into a perilous situation when they hear music emanating from a hatch. In a shocking moment of vengeance, Poppy dons a mask of an unused fifth character, Hooty, and avenges Thadd by killing Karl.
As the climax builds, our protagonists discover Fleegle and Drooper subjecting terrified children to gruesome displays, including the mutilation of Stevie’s corpse. Despite the overwhelming odds, Harley, Zoe, Parker, and Snorky manage to rescue the children by unlocking their chains, prompting a chaotic escape. In a tense showdown, Beth attempts to confront Drooper and Fleegle, but is nearly overpowered until Austin and Harley join forces to defeat the villains.
In a heart-wrenching moment, Snorky sacrifices himself in a heroic altercation with Bingo, and Harley comforts him, affirming his unwavering loyalty. After the dust settles, police arrive to restore order as Austin and Paige ignite a budding romance. Meanwhile, Beth takes the brave step of divorcing Mitch, who, after surviving the earlier carnage, faces his own grim fate as Poppy, now at the wheel of the Banana Buggie, inadvertently runs him over, carrying the remnants of the Banana Splits in tow. As Poppy drives away, a chilling twist reveals Fleegle’s reactivation and maniacal laughter echoing in the studio.
