Four college students – two young men (Randy and Deke) and two young women (Rachel and LaVerne) – go out to swim on a remote Pennsylvania lake during the autumn, when nobody is around. After they swim out to the raft in the middle of the lake, a mysterious oil slick-like creature appears in the water beneath them. Deke ridicules Randy's suspicions that the "oil slick" was chasing the girls, refusing to take the situation seriously until Rachel touches it.
The creature instantly wraps around her arm, pulling her into itself and gradually dissolving her. After the initial panic, Deke decides he could make a swim to the shore, but as he prepares to jump into the lake he steps on a crack on the raft and the creature grabs him by his foot. Unable to free his friend, Randy watches as Deke is slowly consumed by the creature. Now unwilling to risk swimming for shore, Randy and LaVerne take turns watching the creature, which changes positions every now and then - it is either under the raft, or out of it.
One stands while the other one sits. During the course of the night, LaVerne convinces Randy that they should sit together and keep each other warm. He eventually touches one of her breasts, and they end up having sex. During intercourse, LaVerne's hair falls off through the cracks of the raft and the creature absorbs her. The story ends with Randy alone, watching the colors of the creature, until he contemplates ending it all.