Stardate 1704.2: The Enterprise arrives at planet Psi 2000 to pick up a Federation research party from a planet that is quickly disintegrating, but after beaming down to the research facility Spock and Lieutenant Tormolen find all the scientists have died from strange causes. One has been strangled, one sits frozen at his station, apparently without a care in the world, and another is discovered in the shower, fully clothed. After making recordings of the incident the two beam back to the Enterprise, but not before Tormolen is accidentally infected by a water-borne virus that is still alive in the research center. Soon, Tormolen becomes so depressed by what he has witnessed on Psi 2000 that he attempts suicide, only later do die after surgery is performed to repair the minor wound. In trying to stop Tormolen from taking his life Sulu and Riley become infected with the virus, and later while on the bridge Sulu decides that he needs some exercise and abandons his post. When Spock discovers what has happened Riley burst out into irrational speech patterns and is ordered to sickbay. In a short period of time the senior officers conclude that whatever was responsible for killing the scientists on Psi 2000 has come aboard the Enterprise and is now affecting the crew. The only certainty is that everyone infected exhibits some deeply hidden emotion which is unique to every individual. Riley fancies himself a decendant of Irish kings while Sulu becomes a rapier armed swashbuckler who chases crewmembers throught the corridors of the ship. Nurse Chapel declairs her love to Spock who himself can't keep from crying over his past inability to tell his mother that he loved her. Meanwhile, as McCoy frantically searches for the antidote to the virus Riley locks himself into the engine room and shuts down the engines. By the time Kirk and Scotty break throught the locked door, only a few minutes are left to restart the engines, a process that normally takes thirty minutes, and save the ship from spiraling into the atmoshpere of Psi 2000. Kirk, desperate for an alternative, remembers an intermix formula that has never been tried which mixes matter and anitmatter cold. Spock, after shaking off the affects of the virus, helps Scotty restart the engines and save the ship, and in the process they discover a method for travelling back into time.