On the way to pick up Captain Kirk and Science Officer Spock from a peace conference, U.S.S. Enterprise chief engineer Montgomery Scott in temporary command of the ship faces mounting difficulty on his way to the rendezvous - in the form of a menacing alien ship.

The U.S.S. Enterprise crew takes on a Federation historian to investigates an ancient space station which has suddenly appeared in orbit of a planet that the crew has visited before. After they enter the space station, they soon realize that there is much more to the ancient complex than they expected.

The U.S.S. Enterprise crew comes across a planet with a pre-industrial civilization that seems to be sending radio transmissions into space. While rendering aid to a crippled courier ship in orbit of the planet, they wonder if the two situations are related.

While ferrying several ambassadors to a conference on Ivor Prime, the U.S.S. Enterprise crew encounters a Naradan cargo vessel in apparent distress. However, there appears to be more to the ailing ship than meets the eye.