For her bachelorette celebration, Casey Morgan embarks on a journey to Costa Rica accompanied by her friends Kirsten and Jill. During a swim near a picturesque waterfall, an unidentified creature lurking below the surface takes a bite out of Casey, setting off a chain of mysterious events. As her wedding to fiancé Jared looms, Casey is already grappling with her uncertainties about their future together. This inner turmoil is compounded when, after a wild night, she awakens on the beach sans her engagement ring, having spent the evening with a stranger.
Back home, Casey struggles to communicate her desire to postpone the wedding to Jared, especially while living in close quarters in separate apartments within the same building—an arrangement meant to placate Jared's stern mother, Mrs. Kennedy, who also acts as their landlord. As her emotional state deteriorates, physical symptoms begin to present themselves; red marks appear on Casey’s skin, and she finds herself occasionally retching a strange, viscous substance. Matters take a grim turn when a festering pustule erupts on her thigh during an intimate moment with Jared, triggering a series of unexplainable changes.
As if that weren't concerning enough, Cyrano, Mr. Matheson’s dog, outright refuses to join Casey for their usual walks, sensing there’s something terribly wrong. In a bid to tackle her escalating condition, Jill lends Casey a cream for her sores, unaware of how deep the problems run. The situation reaches a critical point when Casey learns she is pregnant, presumably from her encounter in Costa Rica. Acting on advice from a clinic, she begins using over-the-counter medication in hopes of regaining her health, but instead, she becomes the host for a disturbing transformation as she starts laying hundreds of insect eggs, her apartment morphing into a grotesque organic hive.
Mrs. Kennedy's investigation into a foul odor coming from Casey's apartment leads to a shocking confrontation. As she chastises Casey for being unworthy of her son, Casey coughs up an acrid substance that injures Mrs. Kennedy fatally, deepening the horror that surrounds her. As Casey continues to change, she avoids Jared while succumbing to bizarre cravings, consuming her own fingernails and even live cockroaches.
Meanwhile, Kirsten, worried about Casey's absence, confides in Jill, revealing that she had a hand in the theft of Casey's ring during their trip. When Kirsten finally visits Casey, she is horrified by what she's found. Casey, now entrenched in her transformation, refuses to leave her eggs behind, even when Kirsten attempts to call for help and discovers Mrs. Kennedy's lifeless body. In a fit of desperation, Casey inadvertently secreted venom onto Kirsten's face, resulting in her untimely demise.
As the chaos unfolds, Jared's own health rapidly declines, with alarming signs such as blood in his urine. Jill, exploiting the situation, shows Jared incriminating footage of Casey's indiscretion in Costa Rica and seduces him into an affair right outside their building. The heightened tension takes a turn when Casey hears them and, driven by a surge of emotions, screams out, causing them both to expel strange substances simultaneously.
When Jill confronts Casey, the horror escalates as Casey captures her using her now-deadly stinger tail. As she confronts Jill with undeniable proof of her betrayal, a gruesome confrontation ensues, culminating in a tragic and violent showdown with Jared—who, after a fierce struggle, is forced to confront the monster that Casey has become.
In the aftermath, the scene is surreal; one week later, the police and hazmat teams discover the shocking aftermath in both Casey's and Jared's apartments, which have transformed into insect-infested lairs. They uncover a nightmarish reality when a cluster of eggs surrounding Jared hatches, filling the room with swarming bugs.
Ultimately, the story draws us into an unsettling conclusion as Hannah jogs with her friend Joanne, reassuring her about a bite. Little do they know, this tale of decay and horror is far from over.
