Plot

Transporting three Ullian mind-probing historians, the Enterprise crew is stymied when some of its members fall into a coma. Deanna Troi, the first crew member to come through it, cannot recall anything surrounding the incident.

On a mapping journey the Enterprise hosts a party of Ullian 'mind historians', specialists who telepathically retrieve people's long-forgotten memories from many races. Keiko O'Brien and Dr. Crusher enjoy resuscitating pleasant memories, but the overbearing Ullian leader Tarmin's son Jev abuses his abilities on non-volunteers against their code, and enters their thus nightmare-altered memories as a key character, leaving Troi and Riker in apparently unprovoked comas which only resemble the rare Iresine Syndrome, without the associated histamine level change. Then it happens to Crusher the same way, but Troi wakes up after three days, alas with a total black-out. After eliminating any other imaginable causes, Picard has to suspect the Ullians. Troi accepts a memory probe by Jev, who makes her believe it's Tarmin's doing, but Geordi and Data do some research.—KGF Vissers

The Enterprise is transporting three Ullian 'mind' historians. They have the ability to assist people in going deep into their memories to recollect events or occurrences that have long been forgotten. To show how it works, they assist Keiko in remembering her grandmother while she painted pictures and hummed a tune. Not long afterwards, however, Counselor Troi is mentally assaulted and is subsequently found in her quarters in a deep coma. Soon after, Commander Riker and Dr. Crusher also fall into deep comas. Captain Picard suspects one of the Ullians of these attacks but determining who among the three is not going to be straightforward.—garykmcd

On a mapping journey the Enterprise hosts a party of Ullian 'mind historians', specialists who telepathically retrieve people's long-forgotten memories from many races. The Ullians are Tarmin (David Sage), Inad (Eve Brenner) and Jev (Ben Lemon).

Keiko O'Brien (Rosalind Chao) is a crew member on the Enterprise & the wife is transporter chief Miles O'Brien. She has a session with Tarmin enjoy resuscitating pleasant memories. Keiko remembers how her grandmother used to paint while humming a pleasant melody. Tarmin says that the Ullians are planning to construct a vast library to host these retrieved memories from the various species. He believes that the history of civilization resides in the memories of its people. The technique to retrieve memories requires special training and years of commitment to studies and practice. Tarmin is also an overbearing parent to Jev, and likes to demean him in public.

The overbearing Ullian leader Tarmin's son Jev abuses his abilities on non-volunteers against their code. The Ullians never probe memories without the permission of the involved person and Jev breaks this rule.

Jev enters the nightmare-altered memories of his victims as a key character. Deanna had a memory of Will trying to have sex with her and she is saying no, as they are serving on the same ship and this would not be appropriate. Jev enters the memory in place of Will and takes the dream further and tries to force himself on Deanna in her nightmare. This puts Deanna into a coma. Riker questions Jev as Jev was the last person to see Deanna before she went into a coma. Jev is offended at being questioned about his movements and whereabouts by Riker. That night Riker has a dream about how an Ensign died in Engineering when he ordered the bulkheads to be closed while she was still not evacuated. Riker did this to contain a radiation leak, but Jev enters the nightmare and drives the point that Riker killed the ensign by deliberately abandoning her, causing her death. Riker also goes into a coma.

The Comas only resemble the rare Iresine Syndrome, without the associated histamine level change. Beverly investigates further and determines that Deanna's and Riker's brain waves have an Electro-magnetic residue in the thalamus, which is associated with long term memories

Then it happens to Crusher the same way it did to Troi and Riker. Beverly has a nightmare about her visiting her husband's dead body with Picard and Jev enters the dream in place of Picard. Jev also takes the place of her husband's dead body, frightening Beverly and pushing her into a coma. Troi wakes up after three days, alas with a total black-out regarding the incident. After eliminating any other imaginable causes, Picard has to suspect the Ullians. Picard orders Geordi and Data to research all 11 planets that Ullians have cataloged memories from in the past and check for instances of unexplained comas. They find that the Ullians visits coincide with documented instances of Iresine syndrome reported on those planets.

When Picards asks the Ullians to confine themselves, they ask for a chance to defend themselves. Jev offers to probe Deanna's memories to find out what happened to her right before the coma. Troi accepts a memory probe by Jev, who makes her believe it's Tarmin's doing. During the mind probe Jev takes Deanna back to the same memory, but makes it looks like Tarmin took Riker's place instead of Jev. Geordi and Data do some research, as suggested by Picard. They find that only 1 Ullian was present for all incidences of unexplained comas, on all the planets, and that was Jev.

Jev enters Deanna's cabin and tries to force himself on her physically. But Worf enters and knocks him out. Beverly and Riker recover from their comas.