Charlotte, a rebellious 12-year-old girl, lives in a rundown apartment building with her overworked stepfather, Ethan, her mother, Heather, and her baby half-brother, Liam. Often left to her own devices, Charlotte stumbles upon a tiny spider hatched from a strange, glowing object that crash-lands in her great-aunt’s apartment. Fascinated, she decides to keep the spider as a pet and names it Sting.
As Charlotte cares for Sting, she notices that the spider grows at an alarming rate. Initially keeping its size a secret, she soon finds it harder to hide as Sting’s rapid growth leads to strange occurrences in the building. As Sting becomes harder to conceal, the spider’s increasing size and ravenous appetite result in the deaths of pets and even some residents. Meanwhile, tensions between Charlotte and Ethan flare up over unresolved issues regarding her biological father. During one of Ethan’s meltdowns, Sting, now significantly larger, attacks both parents while Charlotte remains unaware, focused on her brother.
The situation reaches a critical point when Sting, now a gigantic and highly dangerous spider, traps several residents within the building. Realizing that Sting has an aversion to mothballs, Charlotte sets out to find her missing family armed with a makeshift solution of mothballs and water. As she attempts to rescue Ethan, they face numerous challenges, including Sting’s growing intelligence and adaptability. The final confrontation occurs in the building’s basement, where Sting launches an attack on the group inside the trash compactor, forcing Charlotte to confront the creature she once considered a pet.