As Anna Maria approaches her 14th birthday, the feelings of insecurity and anxiety become increasingly overwhelming. Her reserved nature is contrasted by her close friend, Paige, whose confidence shines through. On the eve of this momentous occasion, along with Veronica, a free-spirited girl with a charismatic personality, they gather at Anna Maria's home for a slumber party to celebrate not just her birthday but also her father's upcoming wedding. This situation has stirred a whirlwind of conflicting emotions within Anna Maria.
As the night unfolds with laughter and games, Veronica decides to invite Mesmer, a charming yet desperate actor pretending to be a hypnotist. Although the girls harbor some reservations, they ultimately agree to partake in his hypnotic demonstrations.
The following morning, Megan awakes in a bizarre and confusing landscape. With her left eyebrow meticulously shaved, wearing a Jake Ramirez hoodie that brings both confusion and excitement, she surveys the peculiar state of her friends. Veronica sports her outfit backwards and a wild hairstyle, while Paige exhibits the unmistakable marks of a recent scuffle. The most alarming development, however, is Anna Maria's mysterious disappearance, leaving a trail of chaos in her wake. Adding to their confusion is Penny, Anna Maria's younger sister, who seems unaware of the chaos that ensued the night before.
Determined to keep Anna Maria's mother in the dark about their calamity, the girls craft a scheme to find their missing friend. They believe the baby ducks they've discovered belong to their school and enlist Mikey, Paige's older brother, to help them sneak onto their campus. Upon returning the ducks—save for one unfortunate quacker—they stumble upon a shoe belonging to Anna Maria, its paint-speckled design eerily reminiscent of their arch-nemesis, Hillside High. With this new evidence in hand, the girls resolve to uncover the truth behind her disappearance.
Arriving at Hillside High, they are confronted by the marching band, who demand the return of their cherished mascot float, named Hedgie. The band presents a video montage implicating the girls in the theft, leading to negotiations for a plea deal—though none can remember the whereabouts of Hedgie. However, Veronica's astute mind eventually connects the dots to Mesmer's antics at the notorious Onion Festival. Disguised as information-gatherers, the girls infiltrate the festival, only to wind up in an unexpectedly intense onion-eating contest that Veronica dominates. In the ensuing frenzy, they lose sight of Mesmer altogether.
In a moment of desperation, they turn to Jake, who admits he has been in possession of Hedgie all along, albeit for his own reasons. As the girls reclaim their beloved float, they're met with an unexpected twist—the marching band merely wanted back Megan’s outdated phone. This revelation sends a ripple of emotions through the group. Meanwhile, Paige and Veronica take it upon themselves to "rehabilitate" Penny’s dolls in the name of truth, while Megan and Jake can't help but confess their budding feelings for one another.
While Anna Maria's whereabouts remain a mystery, the plot thickens when her note eventually surfaces, disclosing her unplanned hiking adventure that her father was unaware of. The girls discover her peacefully sleeping on a bench, with a humorous reminder of the previous night's antics—a bloody tampon souvenir—which sparks a heartfelt exchange. Anna Maria shares her fears surrounding change and her family's evolving dynamics, eliciting a touching show of support from her friends.
As wedding preparations hit their peak, an unexpected twist arises when Veronica reveals that she has started dating Mikey, leaving Paige feeling betrayed. During a moment intended for Anna Maria's birthday celebration, they are caught off guard by Mesmer once more, brandishing a cryptic phrase that unlocks the memories of the wacky events from the previous night.
In a surprising conclusion during the credits, it comes to light that Paige's accidental nose-thwack caused Anna Maria’s unfortunate nose injury, while Jake was inadvertently swept into the chaos of a midnight prank, solidifying a unique bond with Megan.
