The Enterprise is sent back in time to 1968 to study Earth. While in orbit, the ship inadvertently beams aboard a space traveler, Gary Seven, along with his black cat, Isis. Seven claims to be a 20th-century human raised and trained by unknown and unnamed aliens to prevent Earth from destroying itself. Seven escapes and beams down to the planet below. An orbital bomb is about to be launched by the United States and it is Seven's mission to make sure the rocket explodes somewhere over Asia, thereby frightening the governments into not launching future bombs into space.

Robert Lansing (Gary Seven)

Teri Garr (Roberta Lincoln)
James Doohan (Lt. Cmdr. Montgomery Scott)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Nyota Uhura)
Walter Koenig (Ensign Pavel Chekov)
Don Keefer (Cromwell)
Lincoln Demyan (Sgt. Lipton)
Morgan Jones (Col. Nesvig)
Bruce Mars (Charlie)
Ted Gehring (Policeman)
Paul Baxley (Security Chief)

• In this episode, it appears the Enterprise can just time travel whenever it needs to using the "light-speed breakaway factor." The entire episode takes place in 1968.
  
• Uncertain that Star Trek would be renewed for a third season, Roddenberry used this episode as a test for a new series featuring Gary Seven and Roberta Lincoln. NBC passed on Assignment: Earth and a letter-writing campaign got Star Trek a third season.