In Cleveland, Ohio, high school senior Karen "Wren" DeSantis (Victoria Justice) reflects on Halloween as a festive opportunity for the locals to shed their inhibitions, dubbed affectionately as "the mistake by the lake." Wren aspires to attend NYU, following in her father's footsteps, who always believed that college is a place where one discovers their true self. Her life isn't without its challenges; she has a mischievous younger brother named Albert, who continually finds ways to interrupt her, and is still at the age where he hasn't begun to talk.
Their widowed mother, Joy (Chelsea Handler), is currently dating a much younger man named Keevin (Josh Pence), which adds to Wren's frustrations. While Joy discourages her from pursuing NYU—citing its high costs—she insists that Ohio State is a viable alternative. Meanwhile, Wren's dynamic friend April (Jane Levy) is focused on climbing the social ladder, and Wren's nerdy yet devoted friend Roosevelt (Thomas Mann) is secretly infatuated with her.
The excitement builds when they receive an invite to a Halloween bash thrown by the town's heartthrob, Aaron Riley (Thomas McDonell). Ready for the celebration, Wren decides to dress up as Dorothy from the "Wizard of Oz," while April opts for a kitty costume, and little Albert chooses to be Spiderman. Just as she prepares for the party, her plans are foiled when Joy insists she takes Albert trick-or-treating instead, promising to support Wren's NYU application if she complies.
At the Halloween party run by Keevin’s friend Nate Brueder (James Pumphrey), things unravel unexpectedly for Joy, leading her to stumble into questionable situations. Meanwhile, at a haunted house, Wren and April inadvertently encounter Roosevelt and his best buddy Peng (Osric Chau), but find themselves separated from Albert, whose misadventures lead him to an unexpected alliance with Fuzzy (Thomas Middleditch), a convenience store worker with a vendetta against Jorgen (Johnny Knoxville). As Fuzzy attempts to wreak havoc on Jorgen, he unintentionally drags Albert along for the chaotic ride.
In the unfolding chaos, Fuzzy and Albert attempt a prank on Lara (Abby Elliott), which spirals out of control. In a series of comedic mishaps, it’s revealed that Fuzzy’s schemes lead to some serious trouble, resulting in them being chased by a local bully, Mike Puglio (Patrick De Ledebur). Alongside all this chaos, Roosevelt’s attempts to help Wren by borrowing his parents’ car are met with sexual tension as Wren expresses gratitude with a mere touch, sending him into a frenzy.
Desperate to find her brother, Wren races around town, ultimately tracing Albert’s whereabouts to a Captain Chicken outlet, a fictional pirate-themed fast food chain, where a series of comic escapades ensue. As Wren and her friends navigate the evening's challenges, including Roosevelt’s accidental mischief at the restaurant and an unexpected showdown with bullies, Albert finds himself caught up in another dangerous situation involving Jorgen.
With Wren learning that Albert is in danger, she receives a threatening call demanding ransom for his return. Faced with the difficult task of raising $400, Wren proposes an alternative: a sentimental collectible jacket that belonged to her late father, stirring up some unexpected camaraderie with Jorgen over shared musical interests.
Ultimately, the evening culminates in a dramatic rescue led by Fuzzy, who storms Jorgen's abode during a wild explosion, managing to thwart his plans and rescue both Wren and Albert. The adventure ends as Wren and Albert pay a visit to their father's grave, a touching moment that encapsulates their family bond. Joy eventually allows Wren to blossom, as Roosevelt expresses his feelings, leading to a heartfelt kiss amidst the chaos.
As the night draws to a close, unexpected romances blossom for the other characters too, with Fuzzy discovering a connection with Denise (Riki Lindhome) amidst all the shenanigans. The film concludes humorously with Albert revealing his secretive pranks that had everyone in stitches, solidifying the spirit of Halloween as not just a night of fright, but a celebration of connections, laughter, and love.
