Executive Producer: Gene Roddenberry
Producers: Gene L. Coon + John Meredyth Lucas
Associate Producer: Robert H. Justman
Script Consultant: D. C. Fontana

Story by: Art Wallace and Gene Roddenberry
Writer: Art Wallace
Dir.: Marc Daniels

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Gary Seven: Robert Lansing
Roberta Lincoln: Teri Garr
Policeman: Bruce Mars
Cape Kennedy security chief: Paul Baxley
Cape Kennedy radio voice: James Doohan

Year 1968. The Enterprise has traveled back in time to find out the reason why a rocket, with an atomic bomb attached to it, had failed to work as planned – therefor saving the planet. All of a sudden they encounter a much more powerful transporter beam coming from far away from the galaxy. A man; Gary Seven, and a cat, Isis, have been on their way to earth. A battle begins on board, and Gary Seven, who does not show any reaction to the Vulcan nerve pinch and who knows Vulcans, makes it back to the transporter and beams himself onto earth, continuing his voyage. 

Kirk and Spock follow him fearing his actions might change their history. Following Seven, they arrive an office in New York, where a loyal young secretary fiercely defends her ground, although she's newly hired and totally unaware of her employer's identity. With the help of Scott, who's scanning Earth from the Enterprise, Seven is found at Cape Kennedy sabotaging the controls of a rocket which has got an atomic bomb on board and is about to be launched. Kirk and Spock follow him, but they are detected by the base security and confined. 

All they can do is to helplessy watch as the count down goes on, until Spock manages to nervepinch a guard and to rescue their communicators. They have Scotty beam them back to Seven's office. There, Spock and Kirk find a computer too advanced to operate on. When Gary Seven and his cat, who can change her appearance into a woman (and / or reverse), arrive, Seven convinces both that the rocket needs to be stopped because it would explode on impact. Both trust him and let him operate the computer. He initiates the explosion and the future is not altered. The Enterprise returns to their time.