 When the Enterprise is assigned to observe earths history in 1968, suddenly it intercepts a transporter beam which originates at least a thousand light-years from Earth, bringing aboard a humanoid alien  agent Gary Seven  holding a black cat called Isis, who warns them to step back and let him go to accomplish his mission to save earth; initially phaser-struck down, he manages to beam himself away, actually on a mission to prevent a nuclear rocket being launched at McKinley base because earth is socio-politically not ready for its technological progress. He assumes a classified identity to override a powerful computer, and hits the wrong girl, Miss Lincoln, he mistook for another agent; the computer reports both other agents he seeks are deceased in an accident. Meanwhile Kirk and Spock beam down to investigate if the alien isn t hostile, realizing the risk of changing their own past. When they get on his trail, the girl sees Spock s ears, calls the police and Seven gets away; they must first beam back aboard, then down to the base looking for Seven who overpowers security and sabotages the missile; however they get caught before Scotty locates Seven and beams him up, but he beams himself back grumbling he wasn t finished...