In San Francisco, Elizabeth Driscoll, a researcher, discovers a peculiar type of plant that appears to be a cross between a flower and a pod. After bringing it home, she notices her boyfriend, Geoffrey, behaving strangely, devoid of emotion. Perplexed by his sudden change, she confides in her colleague, Matthew Bennell, who dismisses her fears as fatigue. However, as more residents begin to exhibit the same odd demeanor, Matthew realizes something alarming is happening across the city.

Matthew, Elizabeth, and their friends Jack and Nancy Bellicec become entangled in a horrifying discovery: the pods are part of an alien scheme to replicate and replace humans while they sleep. The group witnesses the duplication process and realizes they are among the few remaining humans yet to be replicated. Struggling to stay awake and maintain their identities, they desperately search for a way to stop the spread of the invasion before they too fall prey to the extraterrestrial threat that looms over humanity.