On April 1, 1973 in Brooklyn, a college student named Bill Tim Martin Crouse takes his pregnant girlfriend Rebecca to a seedy back-alley abortion clinic that also doubles as a brothel. Rebecca is reluctant to go through with the procedure, but the operation proceeds after she is drugged by Big Mama Janet Sovey. The fetus is removed, and Big Mama has her assistant Bertha Antoinette Greene-Fisher dispose of it by flushing it down the toilet. The fetus drifts into the sewer, where it comes into contact with toxic waste leaking from a nearby barrel, gradually mutating into a large, humanoid monster.
After Rebecca regains consciousness, Bertha starts hearing noises from the toilet. When she investigates, the fetus’s tentacle bursts out and rips her head off. Candy, a prostitute Marie Michaels, witnesses the murder and hurries to inform the rest of the staff. They discover that all the doors and windows have been blocked by a strange, fleshy substance, and the phone lines are dead.
The survivors soon witness the monster kill another hooker in the building, and they begin to formulate a plan to escape. Sherman [Gerald Preger], the man in charge of the bordello, is attacked by the creature, and Axel [Frank Rivera], Big Mama’s son and a former prisoner with anger issues, accidentally shoots Sherman to death while trying to kill the monster. Axel then proclaims himself the leader and shoots Candy in the head when she dares to speak back, before telling the remaining survivors that they must kill the monster before they attempt to escape.
On Bill’s suggestion, he, Axel, and a few other survivors venture into the basement, hoping the creature might be hiding in the boiler room. Their theory proves correct, but when Axel tries to kill the creature, he accidentally stabs a fuse box, electrocuting him until his head explodes. Later, Bill and Rebecca manage to break a hole in the front door by tying a rope to a heavy suitcase and using it as a battering ram. When another survivor attempts to escape with it, he finds himself pulled into a strange fleshy environment where the monster attacks and kills him.
Eventually, Bill, Rebecca, and Big Mama are the last survivors. Big Mama goes into the basement to see her son’s corpse, and the creature appears, killing her before bursting into the lobby and killing Bill. With Rebecca as the only one left alive, the monster reverts to the size of a regular fetus and pushes its way inside of her through her vagina. When the police are called to investigate, they find Rebecca alive but in a coma. About a week later, Rebecca, still catatonic, has been taken to a psychiatric ward. The doctors and police officers remain skeptical of her claims, but when a hospital attendant attempts to rape her, the creature erupts from within and kills him.
