Space captain

An alien space captain was in charge of the attempt by a group of radioactive aliens to make peaceful contact with Earth. The Third Doctor was brought aboard his ship after travelling into space in Recovery 7 in search of three missing astronauts. The captain directed him to where the three astronauts, Van Lyden, Michaels and Lefee, were being kept, conditioned to believe they were back on Earth, and then spoke to him over a large video screen, demanding the return of their ambassadors. The Doctor insisted the Earth authorities knew nothing of the arrangement and persuaded the captain to let him return to Earth, although the captain insisted on keeping the three astronauts until the ambassadors were returned.

The captain later contacted the space centre via video link, warning that the aliens had the power to destroy Earth and would use it unless the ambassadors were returned. (TV: The Ambassadors of Death)