Voyager 1, launched by Earth in 1985, approaches the Moon, its disruptive Queller Drive destroying an Eagle, killing Steve Abrams. The drive system, which spews fast neutrons, was already responsible for the deaths of the father of Paul Morrow, and both parents of Jim Haines, and 200 others.
Victor Bergman convinces John Koenig to try saving Voyager, for the information its data banks may contain about alien worlds, despite the danger the ship poses to Alpha. A scientist, Ernst Linden, steps forward with key information about it, and reveals he is actually Ernst Queller.
Meanwhile, three alien ships approach, following Voyager.
Koenig, Bergman, Linden, and Russell board it. The are confronted by the appearance of an image of an alien named Aarchon, who indicates Voyager 1w has extinguished all life on two Sidon worlds. Aarchon condemns both Alpha and Earth to be "extinguished," despite the Alphans arguing it was unintentional, and that revenge is not the answer.
Linden nearly faints from shock and guilt, but later sneaks out of Medical Center, boards Voyager, and launches it towards the Sidon ships. He argues with Aarchon that as creator of the drive that caused all the damage, he should bear sole responsibility. Aarchon does not alter his stance, so Linden uses the drive and a self-destruct mechanism to destroy the Sidon ships and Voyager together.
A troubled Jim is then given the Voyager's data bank to work on.

Though Ernst Linden reveals he is Ernst Queller, to John and Victor, and the latter mention it to some officers, including Paul, it is kept quiet to the rest of the base, including Linden's assistant, Jim. Jim, however, becomes suspicious of his mentor when Linden builds a device to send the proper signals to remotely shut down the drive, and Linden makes statements about how it works, the latter bringing Haines to decide only the Queller Drive's inventor could do this. Jim confronts, assaults, and injures Ernst, which also results in the device being damaged, and Jim being reprimanded by the commander. Though in pain, Linden returns to duty, effects repairs, and gets the Queller Drive shut down.
1985 is mentioned as being 15 years before. Either the episode is set in 2000 (by Alphan timekeeping), which seems likely; or perhaps the character (Victor) was rounding a little.