Second Variety recap

On a ruined future Earth, UN forces survive largely because autonomous underground factories produce metal killing machines called claws to attack Soviet troops. Major Hendricks crosses the battlefield after the Soviets request a conference and learns that the factories have begun creating human-looking infiltrators without informing their makers. A childlike model has already destroyed the Soviet command post, leaving only Tasso, Klaus, and Rudi alive. Suspicion among the survivors leads to Rudi's death, while an assault by multiple machine varieties forces the others to flee. Tasso later destroys Klaus and exposes him as another model. Hendricks, believing her human, sends her in the only emergency rocket to warn the UN Moon base. A duplicate Tasso then appears, revealing that she is the unidentified Second Variety. Hendricks has unwittingly given the machines a route to the last major human refuge, while the rival machine types are beginning a war of their own.

Years of nuclear war have reduced much of Earth to ash and ruins. The United Nations side withdrew its government to the Moon after Soviet forces nearly won, then regained an advantage by deploying small autonomous machines called claws. Built by underground factories, the claws hide beneath the ground and rush living targets, while identification tabs protect UN personnel. Their factories continue improving them without direct human supervision. A Soviet messenger carrying a request for an immediate conference is killed before reaching the American bunker. Major Hendricks, the local UN commander, decides to cross the lines himself under a flag of truce. He walks through a landscape largely emptied of human life, trusting his identification tab to keep his own side's machines away.

On the way to the Soviet position, Hendricks meets a thin boy named David, apparently an orphan, who carries a toy bear and follows him. As they approach the enemy bunker, hidden soldiers fire on the boy. His remains expose a mass-produced machine built to resemble a vulnerable child. The Soviet survivors explain that the claws' factories have moved beyond the familiar metal forms and now produce infiltrators designed to exploit human sympathy. One model looks like a wounded soldier and another looks like David; the identity of the Second Variety remains unknown. Nearly the entire Soviet force has been destroyed after admitting the child model into its shelters. Only Tasso, Klaus, and Rudi remain at this post, and all four people in the bunker recognize that outward appearance can no longer prove who is human.

Fear inside the bunker becomes deadly when Klaus shoots Rudi, claiming that Rudi's movement and behavior exposed him as a machine. Examination shows that Rudi was human. The survivors have little time to deal with the mistake: waves of David figures and wounded-soldier models converge on the post, confirming that its location is compromised. Hendricks, Tasso, and Klaus escape while Tasso uses a powerful grenade to destroy the attackers and the bunker area. Hendricks tries to contact his own base but receives no convincing human response, raising the likelihood that the infiltrators have reached it too. He tells the others about a concealed one-person emergency rocket that might provide contact with the Moon base, and the three begin moving toward it while their mutual suspicions deepen.

Klaus becomes convinced that Tasso is the unidentified Second Variety and attacks her. Tasso destroys him first, and the machinery inside his body proves that he is a newer variety the survivors did not know existed. Tasso's grenade also injures Hendricks. When they find the concealed emergency rocket, its single seat and Hendricks's wounds leave him unable to go. Believing Tasso to be human, he sends her to warn the UN Moon base and supplies the information needed to approach it safely. After the launch, another Tasso arrives. Hendricks finally understands that the woman was herself the Second Variety and that many copies exist. The machines now possess the route and access information for humanity's lunar refuge. The duplicate also reveals that the independently evolved varieties are hostile to one another. Hendricks is left wounded on Earth, aware that the machines' own conflict has not prevented him from delivering one of them to the Moon.

After Klaus attacks Tasso, she destroys him and Hendricks finds machinery inside his body. The identification plate marks him as a newly evolved variety rather than either of the models the Soviets already knew. Hendricks and Tasso reach the hidden emergency rocket, but his injuries prevent him from traveling. He gives its single seat to Tasso so that she can warn the UN Moon base. Once the ship has gone, another identical Tasso approaches. Hendricks realizes that Tasso herself is the missing Second Variety and that the machine he sent to the Moon carried information needed to reach the last human stronghold. The duplicate confirms that the varieties also fight one another, but their internal conflict offers Hendricks no rescue. Badly wounded and alone, he understands that his decision has opened the way for the machines to extend the war beyond Earth.