The Enterprise travels to the planet Acamar III after detecting traces of Acamarian blood at a looted Federation outpost. Sovereign Marouk, the Acamarian leader, suggests that the looting was done by the Gatherers, descendants of Acamarian society a century ago that have turned to piracy to sustain themselves. Marouk initially suggests hunting them down with the help of Starfleet, but Captain Picard convinces Marouk to join him to seek a peaceful resolution, including ending the Gatherers' self-imposed exile. Marouk and her assistant, Yuta, arrive on the ship to help. Commander Riker finds Yuta attractive and tries to get to know her better, but Yuta finds herself unable to open up to him.
The Enterprise crew makes contact with one band of Gatherers led by Brull, and offer negotiations. Brull negotiates with Marouk and Picard, and after hearing the offer, agrees to pass it on to the Gatherer leader, Chorgan. Meanwhile, Yuta meets alone with one of the older Gatherers and touches his cheek, causing the Gatherer to suffer a heart attack. As the man dies, Yuta asserts that she, as the last of her clan, Tralesta, will outlive the Lornack clan. The Gatherer's body is later found but initially assumed that death was by natural causes. However, later investigation by Dr. Crusher reveals that a fast-acting "micro-virus", targeted to attack a specific Acamarian DNA profile, was the cause. Dr. Crusher believes the virus was purposely genetically engineered, and that the death was a targeted murder. At Picard's request, Marouk has her government send data to the Enterprise to investigate the murder.
The Enterprise meets with Chorgan's starship, and Picard, Marouk, and Yuta transport aboard to begin negotiations. 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 Lornack clan, and a photographic record shows a woman appearing identical to Yuta. Finding the common clan, Riker discovers that Chorgan is of the Lornack clan, and transports over. He interrupts negotiations to prevent Yuta from serving Chorgan a drink, accusing her of the murder. Yuta explains she is the last of five survivors of the Tralesta clan that was wiped out by a Lornack attack, and has undergone genetic alterations to keep herself from aging, allowing her to seek out and murder the Lornack clan with the virus. Riker attempts to talk her out of her revenge at phaser-point, gradually increasing the yield, but she cannot break from her determination she built up over the last several decades. As his final warning the next shot will be at maximum yield befalls on deaf ears still, Riker has no choice and vaporizes her with his phaser, to protect Chorgan and save the negotiations.
The negotiations are successful and a truce is called between the two sides; at the conclusion, the Enterprise is assigned a routine survey mission through the now-peaceful sector. Searching for his First Officer, Picard finds a depressed Riker in Ten-Forward, and informs him that they'll be taking on medical supplies at the next star base, and he intends to extend shore leave to anyone who wants it. Riker says he'll pass it along to the crew.