In this episode, the crew of the Enterprise tries to negotiate an end to raids launched by a group called the "Gatherers," but a murder threatens to prevent peace.
The Enterprise travels to the planet Acamar III after detecting traces of Acamarian blood at a looted Federation outpost.
However, later investigation by Dr. Crusher reveals that a fast-acting "micro-virus", targeted to attack a specific Acamarian DNA profile, was the cause.
At the same time, the Enterprise crew receives the database from Acamar, and find that fifty-three years earlier, another Gatherer suffered a similar heart attack - he too was from the Lornak clan, and a photographic record shows Yuta was present, and clearly has not aged a day.
Riker attempts to talk her out of her revenge at phaser-point, gradually increasing the yield with each successive shot, but she cannot break from her desire for vengeance that she built up over the last several decades.
A truce is negotiated between the two sides; at the conclusion, the Enterprise is assigned a routine survey mission through the now-peaceful sector.
On June 6, 1995 "The Defector" and "The Vengeance Factor" were released on LaserDisc in the United States by Paramount Home Video.