Three beings, disguised as human males wearing leather jackets, are part of an advance alien invasion force, sent to Earth to infect city water reservoirs with bacteria. Calling themselves "Scott", "Steve" and "Fred", they rent a house in a suburban American town where they can keep in visual contact with their planet's leader while setting the plan in motion.

The alien plan is to kill all humans and domestic animals. Their own race needs room to expand, and this is an easy way to take over the Earth for themselves. The excuse given by the alien leadership, for the extermination, is that humans are violent and hateful and therefore deserve to be destroyed. The youngest of the three aliens, called Scott, falls in love with the girl who lives next door to their rented house, Ellen Tillman. Scott tries to convince the leader of his world that not all men are evil and that love exists among people of the Earth, but to no avail.

Scott tries to convince Ellen to run away with him so that she will escape death, but Ellen is so terrified of his claims, she believes him to be mentally disturbed. Ellen's father calls the local sheriff to take Scott away. The deputy who answers the call turns out to be part of the invasion force, intending to punish Scott for his defection. The Tillmans comfort their daughter not realizing they are doomed from the upcoming alien invasion.

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