When the Enterprise is assigned to observe Earth's 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 mistakes the wrong girl, Miss Lincoln, 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.