The Enterprise comes to the aid of a Federation transport under attack by two ships, saving its crew and passenger, Lumerian ambassador Ramid Ves Alkar. Starfleet orders the Enterprise to transport Alkar the rest of the way to the Rekag/Cironi system, where he is to mediate negotiations between the two planets. Alkar is accompanied by an old woman named Sev Maylor whom he identifies as his mother. Counselor Troi and Alkar form a friendship during transport, but this causes Maylor to become bitterly hostile towards Troi. Soon, Maylor succumbs to some unknown fate and dies; Alkar appears unmoved by the loss but still requests a funeral ceremony along with Troi, having them touch "funeral stones" as part of the process. Dr. Crusher requests to autopsy the body due to high levels of neurotransmitter residue, but Alkar denies it, asserting it would violate Lumerian custom.
As the Enterprise continues, Troi starts acting in an increasingly irritable and jealous manner initiated when Alkar refused her seducing advances. Troi attempts to seduce other members of the crew, putting her relationship with Commander Riker at odds. Troi also shows signs of advanced aging. When they arrive at Cironia, Troi becomes even angrier when she discovers Alkar working with Liva, another female in Alkar's delegation that had already arrived on the planet. After Alkar refused to let Troi come with him to the planet, Troi attempts to attack him with a knife, but security restrains her and takes her to Sick Bay.
The visibly aged Troi shows similar high levels of neurotransmitter residue as Dr. Crusher found in Maylor's body, and with this new evidence, Picard allows her to perform the autopsy on Maylor. Dr. Crusher finds that Maylor was only thirty years old, and had no genetic relation with Alkar. Picard confronts Alkar on the planet about this finding. Alkar reveals to Picard that he has the ability to channel his negative emotions into another person, a "receptacle", which allows him to be clear-minded and level-headed as a negotiator. Alkar is aware this causes accelerated aging and death of the receptacles within a few years, and had not foreseen Troi's rapid symptoms when he chose her as his next receptacle.
With Alkar showing no remorse for his actions, Picard and Dr. Crusher devise a means to rescue Troi. Dr. Crusher induces deathlike symptoms in Troi, which breaks the link with Alkar. Alkar returns to the ship to acknowledge Troi's death, and then seeks out Liva, looking to make her the next receptacle. Just as Alkar starts the process, Dr. Crusher revives Troi and purges the neurotransmitter residue from her system; this has an immediate impact on Alkar as he tries to finish the channeling ritual. Picard has Liva beamed out of Alkar's quarters, leaving the man alone to suffer from years of repressed emotions. He ages quickly and dies. Troi is able to make a full recovery, gaining her former youthful appearance and her original personality.