San Francisco is slowly covered by strange looking spores that transform in to beautiful flowers. Unknown to the unsuspecting public, these innocent looking flowers contain alien capsules which produce a web like form to capture and replicate their bodies during the night. The victims still keep their human form as they carry out the work of their alien captors to control mankind. It is not too long before there are only four humans left in San Francisco. Matthew Bennel (Donald Sutherland) is the Department of Health inspector who leads this small group in an effort to regain control of their city, together with his friend Elizabeth Driscoll (Brooke Adams). Their battle against the alien invaders takes an unexpected turn when they come across psychiatrist Dr. David Kibner (Leonard Nimoy).