Enterprise conveys a delegation of Ullians to Calder IV. The Ullians are telepathic historians who conduct their research by retrieving long forgotten memories.
After helping Keiko O'Brien recall a memory from her childhood, Tarmin, leader of the Ullians, asks for further volunteers, pointing out to Chief Medical Officer Beverly Crusher that he knows she is thinking about her first kiss. Tarmin is rebuked by his son Jev, who reminds him that he should not probe someone's memory without permission.
Deanna Troi asks whether all Ullians can read memories. Tarmin tells her that it requires years of training. He recalls Jev was unable to read some Gentons, while Tarmin did so easily. This upsets Jev, who leaves the room.
Troi follows Jev, who explains to Troi that his father enjoys demeaning him in public. Deanna replies that she can relate, as she has an overbearing parent. Deanna bids Jev goodnight; the two are attracted to one another.
In her quarters Troi recalls a romantic interlude she had with Commander Riker. In her flashback, Riker becomes forceful with her. She tries resisting him, and suddenly Riker is replaced by Jev; whom she also sees in her cabin briefly. She screams and collapses.
Troi is discovered in a coma. Doctor Crusher cannot find any reason for her condition, and decides to withhold more extreme treatments until she can isolate a cause.
Riker questions Jev, known to be the last person to see Deanna, and asks that the Ullians allow Crusher to examine them for any harmful toxins or pathogens. Jev agrees to these exams. Later, Riker experiences the same kind of flashbacks as Deanna, and also becomes comatose.
Beverly examines the Ullians and does not find any harmful agents. She meets with Picard and Lieutenant Worf and tells them that she found a trace of electropathic activity typical of a rare neurological disorder, Iresine Syndrome. Worf suggests putting the Ullians in quarantine since Riker and Troi were healthy before they came aboard, but Picard feels this would be premature. Crusher notes that the residue she discovered was in the region of the brain that involves memory function.
Meeting with the Ullians, Picard requests further examinations. Saying they have nothing to hide, the Ullians agree. Beverly examines Keiko, but finds nothing amiss. Crusher continues researching the problem, but experiences the same type of flashbacks Deanna did, and she screams as Jev invades her mind.
Picard orders Data to assume the investigation, asking him to check medical records of planets the Ullians visited while doing their research, looking for unexplained comas.
Troi regains consciousness but cannot remember anything about the attack. Picard visits the Ullians again, requesting that they confine themselves to quarters until the mystery is solved. Jev asks to probe Troi's memories to help her remember what happened when she fell into her coma.
Geordi and Data find no unexplained coma cases, but La Forge realizes that the physicians on those worlds might not have been as thorough as Crusher and attributed the comas to other causes. Repeating the search for cases of Iresine syndrome, they find two cases occurred on Hurada III while the Ullians were present.
Jev begins probing Deanna's memories, with the other Ullians as well as Picard and Worf present. Deanna remembers that right before she fell into the coma she had memories of someone touching her hair, and recalls that in her flashback she was with Riker. Finally she recollects that Riker was replaced by Tarmin.
Jev meets with Picard, informing him that he has contacted his home world and the authorities will support prosecution of Tarmin. Picard replies that an invasion of one's memory is not a crime in the Federation, but Jev replies that such incursions were outlawed on his world, and the punishment is severe. Jev tells Picard that even though Tarmin maintains his innocence, his telepathic activities are being monitored and he will not assault anyone else.
Geordi and Data discover that Tarmin was not part of the Ullian delegation on two of the worlds where there were coma victims. Jev comes to Deanna's cabin to tell her goodbye and apologize for all his father put her through. Deanna sees that he is upset and asks if he wants to discuss the matter. Jev becomes agitated and initiates another mind probe. Troi finds the flashbacks starting again, and realizes that it was Jev who assaulted her. Jev begins attacking her physically. The door opens and Data, Worf and other security officers enter. Worf fends off Jev and Data explains that he and Geordi discovered Jev was the only Ullian present during all of the onsets of coma.
The Enterprise sets course for the Ullian home world; on the way Riker and Crusher awaken. Tarmin tells the affected officers that physicians from his home world will help them with their healing. He adds that such attacks are almost unimaginable to the advanced Ullian society; his people thought they had moved beyond such ways. Picard points out that humans were also violent and savage, and that even though they have evolved, they still contain the seeds of that violence. He adds that this violence is still capable of consuming them, as it apparently did Tarmin's son.