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While on her bachelorette party getaway, Casey, the bride to be, gets a seemingly harmless bite from an unknown insect. After the trip, Casey discovers that everything can change with a single bite.

While on her bachelorette party getaway, Casey, the bride to be, gets a seemingly harmless bite from an unknown insect. After returning home with cold feet, Casey tries to call off her wedding but before she's able to, she starts exhibiting insect like traits. Between her physical transformation and her wedding anxiety, Casey succumbs to her new instincts and begins creating a hive that not only houses her translucent eggs, but feeds on the flesh of others. As her transformation becomes complete, Casey discovers that everything can change with a single bite.—unknown

For her bachelorette party, Casey Morgan goes to Costa Rica with her friends Kirsten and Jill. Something unknown swimming underwater bites Casey while she and Jill swim near a waterfall.

Already uncertain about her upcoming marriage to fiancé Jared, Casey returns home with additional guilt after spending a drunken evening with a man met in Costa Rica and waking up on the beach without her ring. However, Casey is continually unable to tell Jared that she wants to postpone their wedding. Casey and Jared live in separate apartments in the same building to appease Jared's disapproving mother Mrs. Kennedy, who is also their landlord.

Red marks start spreading all over Casey's body and gradually worsen. Casey also begins occasionally vomiting a viscous substance. A festering pustule explodes on Casey's thigh while having sex with Jared. Cyrano, a dog belonging to neighbor Mr. Matheson, refuses to go for his regular walk with Casey.

Having seen a dermatologist for her own bug bites, Jill lends Casey a cream for her sores. Casey discovers she is pregnant, presumably from the man in Costa Rica. Casey also calls a clinic about her condition and acts on phone advice to purchase over-the-counter medication.

Casey's mutation continues as she starts laying hundreds of insect eggs. Her apartment transforms into an organic hive.

Mrs. Kennedy comes over to investigate complaints of a smell coming from Casey's apartment. As Mrs. Kennedy berates Casey for not being good enough to marry her son, Casey coughs up an acidic substance that burns Mrs. Kennedy's face and kills her.

Casey and her apartment continue transforming as she also continues avoiding Jared. Casey eats her fingernail as well as a live cockroach.

Kirsten talks to Jill about why they haven't seen Casey lately. It is revealed that Jill, who is secretly obsessed with Jared, stole Casey's ring while they were in Costa Rica.

Kirsten comes to Casey's apartment and is horrified by the scene. Kirsten implores Casey to leave with her, but Casey insists that she cannot abandon her eggs. Kirsten tries hiding in the bathroom to call 911, but drops her phone in the water when she sees Mrs. Kennedy's corpse in the bathtub. Casey puts her hand over Kirsten's mouth to quiet her and a blue substance secretes from Casey's palm into Kirsten's face unconsciously. Kirsten dies.

Jared starts peeing blood. Jill shows Jared video footage of Casey with the other man in Costa Rica and tells him that they had sex. Jill seduces Jared and they start having sex in her car while parked outside Jared and Casey's apartment building.

With her heightened senses, Casey hears Jared and Jill having sex and screams. Jared and Jill suddenly spit up a strange substance. Jared exits the car and Jill drives away.

Jill comes to Casey's apartment and sees Kirsten's body hanging from a wall, which is now covered in an organic substance. Casey captures Jill using her newly-developed poison stinger tail. Jill pleads for mercy before Casey shows her video footage proving that Jill purposefully allowed the man in Costa Rica to take advantage of Casey while she was drunk. Casey kisses Jill to put pus in her mouth and forces her former friend to scream for Jared.

Jared races to Casey's apartment. Casey slits Jill's throat in front of Jared before attacking him. Jared wrestles Casey to the ground, but she stings him with her tail. Jared grabs a wooden furniture leg and thrusts it into Casey. He then repeatedly stomps Casey's head with his foot and kills her.

Jared stumbles back to his apartment while spitting up pus. He collapses before he can dial 911.

One Week Later - Police and a hazmat team process the scene. Jared's apartment has transformed into an insect lair similar to Casey's. Numerous eggs surrounding Jared's body suddenly hatch and strange bugs fly everywhere.

Hannah jogs with her friend Joanne. When Joanne is bit by something, Hannah calms her by saying it is only a bug bite.