A huge, alien structure resembling an inverted cone, appears in the woods outside a small rural town. Sent from Washington to investigate the origin of the mysterious object, a team of investigators discovers that intelligent parasites from inside the "cone" can attach themselves to humans' nervous systems and control their minds, taking control of the authorities and workers, making communication with the outside world impossible, and leaving the responsibility of stopping the invasion up to seven people who have thus far been able to avoid possession by these creatures from parts unknown.

Beautiful alien Amazonian women plan to conquer the world using an army of vegetable monsters. Dim-witted privates Philbrick and Penn bumble into a cave in search of atomic activity but collide instead with fierce carrot-topped tree mutants and their leaders, the 7-foot space sirens Prof. Tanga and Dr. Puna. This lavishly low-budget sci-fi romp has the bodacious aliens planning to overrun Earth with their vege-men army, but first they want the G.I.s to explain the meaning of love.