Tarmin was an Ullian telepathic historian and the most proficient of his group. He had at least one son, Jev. Tarmin was trained in the art of telepathic memory retrieval. He considered himself an "archaeologist of the mind," and was proud of his ability. Tarmin had a large ego and was not ashamed of embarrassing his son in public, generally finding it amusing.
Tarmin spent several years with Jev and another Ullian, Inad, gathering memories from eleven planets in eight star systems for use in a large library of memories. Many were freely willing to participate, but Tarmin was not above using encouragement to have others agree to the process. During this process, Jev spent two days with a contingent of elderly Gentons and was unable to recover anything of substance. Tarmin spent one hour with the group, and retrieved memory fragments from the Gentonian trade wars. Other planets visited by Tarmin during this time were Melina II and Hurada III.
The USS Enterprise-D transported Tarmin, Jev, and Inad to Kaldra IV in 2368. Tarmin helped Keiko O'Brien retrieve a memory from her early childhood of working with her grandmother while she did ink brushing. No other Enterprise crewmembers volunteered for a memory retrieval, despite Tarmin's requests.
He eventually accepted the crew's refusal, but his eagerness to probe their minds later caused him to become implicated in a series of comas which occurred aboard the ship. The illnesses were initially blamed on Iresine syndrome, but when scans revealed that there were no sources of the disease aboard the Enterprise-D, the telepathic abilities of the Ullians were placed under suspicion. Tarmin was upset and felt that the Ullians were being unjustly accused, especially after an examination of Keiko O'Brien revealed no harmful side effects from the memory probe. He refused to take part in any further investigation.
In actuality, the comas had been caused by Jev, perpetrating telepathic memory invasion. When Counselor Deanna Troi, one of the affected officers, awoke from the coma, Jev volunteered to probe her mind and recover the memory of the invasion. Jev was able to manipulate Troi's memory to make it appear as if Tarmin was responsible. Captain Picard subsequently placed Tarmin under arrest.
Further investigation by Enterprise-D crew revealed that cases of Iresine Syndrome were also reported on planets visited by the Ullians, although Tarmin was on the Ullian homeworld when the cases on planet Nel III occurred. Jev was on the planet when the comas occurred, leading to the conclusion that Jev was in fact responsible. He was arrested shortly before he assaulted Counselor Troi for a second time.
Tarmin was released as the Enterprise-D returned Jev to the Ullian homeworld. Tarmin contacted the best physicians on his homeworld, and requested that they meet with the affected Enterprise-D crewmembers upon the starship's arrival. Tarmin was dismayed that his son could be capable of such violence. The whole experience caused Tarmin to reevaluate his attitudes toward his son, and he became much less pompous. (TNG: "Violations")