The Enterprise is attacked and boarded by the unusual space probe Nomad, which Spock identifies as the combined remains of an alien robot and an Earth exploration probe. Nomad's purpose - a confused mix of aliens' orders and instructions from Earth - is to seek out and sterilize all impurities, including imperfect beings like humans. The only thing preventing Nomad from obliterating the Enterprise and everyone on board is the similarity between the name of Nomad's creator and Captain Kirk, and Kirk must try to play that role as best he can while figuring out how to get rid of Nomad.
