One of the best episodes in the series

Moonbase Alpha encounters an unknown spacecraft and when they realize what it is, it is too late to save one of their own ships. The unknown craft is Voyager One, a probe launched from Earth years before. Its' mission was to explore the universe and gather data about other stars and planets.
Voyager One was equipped with two propulsion systems, a conventional rocket system for maneuvering in planetary systems and an atomic engine that allowed it to travel interplanetary distances. The atomic engine, called the Queller Drive after the designer, is very destructive and that is what destroyed one of Alpha's ships. When Voyager was launched, the Queller Drive activated too soon, killing several hundred people, including the relatives of some current residents of Alpha.
Since Voyager One is going to pass close enough to Alpha to kill the people on it, they are faced with a choice. Either destroy the probe or deactivate the Queller Drive. Professor Bergman argues that they must make every effort to deactivate the probe, as it is important for them to extract the scientific data. John Koenig, the leader of the Alpha colony, agrees and they attempt to deactivate the probe. When their first efforts fail, Alpha resident Dr. Ernst Linden informs Koenig that he is in fact Ernst Queller and that he can deactivate the probe. He succeeds and the probe lands on Alpha.
However, space ships from another civilization are following the probe and they contact Alpha, telling them that the Queller Drive killed everyone on two of their planets and that they will destroy Alpha in revenge. Queller then gets in Voyager and uses the Queller drive to destroy the alien ships, saving Alpha.
This is an excellent story, with some very good special effects that wear well even after thirty years. The only flaw is the extent of the hostility that people have against Ernst Queller. The premature activation of the Queller Drive was a technical malfunction that cannot be blamed solely on Queller.

great action packed episode,must see for all sci fi fans

one of the first seasons best shows. special effects are great as well as story line.....this series should be reproduced for the new generation of people who like sci fi...lets start a letter writing campaign to itc,its distruibtor.