In 2265, Tormolen and Lieutenant Daniel Alden met up with Hikaru Sulu in the Enterprise's botany lab, and then proceeded to attend Lee Kelso's memorial service in the ship's chapel. (TOS - My Brother's Keeper novel: Constitution)
In 2266, Tormolen accompanied Lieutenant Commander Spock to Outpost ULAPG42821DB on Psi 2000, where the discovered that the researchers had killed themselves after undergoing polywater intoxication. During the mission, Tormolen removed the glove from his environmental suit and absentmindedly touched a console which had a drop of blood on it. He carried the polywater back to the Enterprise and gradually infected the crew.
As the infection took hold of Tormolen, he began to express doubts about his life in space and began increasingly short tempered. During a meal with Lieutenants' Sulu and Kevin Riley in the recreation room, Tormolen stabbed himself with a knife in front of his friends and crewmates. Although, Tormolen made it into surgery in plenty of time, Doctor Leonard McCoy's attempts to rescue Tormolen failed and he died on the operating table. McCoy became dismayed that such a minor injury could have killed him, remarking that it was as if Tormolen lost the will to live. (TOS novelization: The Naked Time)
In an alternate timeline encountered by the Enterprise crew shortly after in which Captain Kirk died in 2247, the Enterprise didn't visit Psi 2000 and Tormolen didn't die. Tormolen remained aboard the Enterprise until the end of the ship's five-year mission in 2270. Following the outbreak of hostilities between the Federation and the Gorn Hegemony, Tormolen was assigned to the USS Delphi. (TOS - The Janus Gate novel: Present Tense)