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Will (Billy Mumy), Dr. Smith (Jonathan Harris), and the robot encounter a super-powerful android, IDAK (short for Instant Destroyer and Killer) Alpha-12 (Don Matheson), who has been dispatched to destroy a runaway android. Having been damaged by his interstellar journey, and a little disoriented in dealing with humans, IDAK accepts their help in recovering his full strength and also their assurances that there are no androids among the Robinson party, just humans and one robot. But they're wrong -- Verda (Dee Hartford), the female android introduced earlier in the episode "The Android Machine", has returned to them. A policy change at the Celestial Department Store resulted in an order for all androids of her type to report for destruction, but she refused and ran, transporting herself back to the Robinsons' planet -- except that she's changed, and she looks and acts almost completely human. At first, unable to convince IDAK Alpha-12 to abandon his programming, she is able to convince him that she is human, and enough of what she shows him about being human rubs off, that he is able to resist his programming to kill her. When the CDS sends another, much more powerful IDAK Omega-17 model (Dawson Palmer), however, the first IDAK struggles between the new impulse for survival and the self-sacrifice that he sees in Verda, and finally joins together with the Robinsons to combat the threat.