After finding traces of Acamarian blood at a looted Federation outpost, the Enterprise goes to Acamar III and runs into a group of Acamarian nomadic pirates known as the Gatherers â descendants of people who had separated themselves from Acamarian society a century before, during a time of rampant clan warfare.
Now, the Acamarian government wants to negotiate an end to the Gatherers' self-imposed exile and invite them to return to their home world. The Gatherers are skeptical, but they agree to listen to the proposals of the Acamarian leader, Sovereign Marouk. Marouk comes aboard the Enterprise, along with her personal servant (a beautiful and charming young woman named Yuta).
After the first brief meeting between Marouk and a group of Gatherers, Yuta privately confronts an elderly Gatherer and touches him on the cheek â whereupon the man promptly has a massive heart attack. As he draws his last breath, Yuta tells him that although she is the last of her line, her clan (Tralesta) will outlive the dying man's clan (Lornack). No one else is aware of any of this, and when the man is later found, everyone assumes he simply died of old age.
Aboard the Enterprise, Riker is strongly attracted to Yuta. She tries to return his affection but finds herself unable to open up to him (either physically or emotionally).
In Sick Bay, Dr. Crusher has been examining the body of the Gatherer and discovers that his heart attack was caused by a fast-acting "micro-virus" that would attack only Acamarians with a very specific DNA profile. Other Acamarians, with different DNA profiles, could come into contact with this virus â or even be carriers â without suffering any ill effects. Such an organism couldn't possibly have evolved naturally, Crusher insists; it must have been genetically engineered, which means the man was murdered.
As the Enterprise meets the starship of Chorgan, the Gatherer Leader, a large download of information from the Acamarian planetary database arrives via subspace radio. Commander Data studies this material and finds a record of a similar mysterious death â another Gatherer from the Lornack clan who had a massive heart attack during his trial, 53 years ago. A photograph of the man, being led from a session of his trial, is found in the database, and in the background of the old photo is a young woman who appears identical to Yuta.
Riker beams over to the Gatherer ship and interrupts the negotiations just as Yuta is about to take a cup of brandy to Chorgan (who is also of the Lornack clan). Confronted by the new revelations about her past, Yuta admits that she is the last of five survivors of an attack on the Tralesta clan by the neighboring Lornack clan. Although that attack happened 80 years ago, Yuta hasn't aged a day since then because she was genetically altered to enable her to seek out the remaining Lornacks and kill them all, one by one, with the specially engineered virus she carries.
Riker tries to appeal to Yuta's kinder nature and begs her to abandon her quest of vengeance, but the woman is unable to shake off a century of single-minded determination. Even stun blasts from Riker's phaser fail to stop her, and in the end, he has no choice but to vaporize Yuta with his weapon's highest setting in order to stop her from lunging at Chorgan. The peace talks are saved, and the greatly relieved Gatherer official offers Riker his deepest thanks for saving his life, but the Enterprise's second-in-command is clearly too depressed by what he was forced to do to respond to the man's gratitude.