Dr. Harrison
John Bellah

Lt. Kevin Riley
Bruce Hyde

Joe Tormolen
Stewart Miss

Lt. Brent
Frank Da Vinci

Its latest assignment takes the U.S.S. Enterprise to retrieve a science team from the surface of Psi 2000, a dying planet, and observe its disintegration. However, on entering orbit, sensor scans show the members of the team to be dead. Mr. Spock and crewman Joe Tormolen are transported down to their camp in environmental suits to investigate the cause and discover that the behaviour of the team before they died had been very strange.

One is found to have been taking a shower with his clothes on. While Spock downloads the computer records Tormolen has a look around. For some reason he removes a glove and, on touching a wall is unknowingly infected with an alien virus. After their return Tormolen starts to act strangely, eventually attempting to stab himself with a knife but is restrained from doing so too severely by Kevin Riley and Sulu.

Joe is taken to sickbay with minor wounds that are not bad enough to account for his death shortly afterwards. In touching Tormolen, Riley and Sulu have themselves become infected and soon both also start to exhibit strange behaviour, a condition which spreads rapidly around the ship. Stripped to the waist, Sulu terrorises the corridors of the Enterprise with a sword while Riley, behaving as if drunk, takes over control of the ship from the main engineering department.

The Enterprise is still in orbit around Psi 2000 and is threatened with destruction unless it moves off before the planet disintegrates - a fact that Riley chooses to ignore. His requests and announcements over the ship's intercom become ever more manic and outlandish. In desperation, Scotty and a team of engineers try to cut through a bulkhead to reach him without severing vital relays while, in sickbay, McCoy races against time to try and find a cure. Scotty's team manages to get in and remove Riley but find that he has shut down the main warp engines, which they need in order to escape.

Kirk and Spock decide to try something that has only ever been theorised about - a controlled implosion of cold matter and anti-matter. The huge gamble comes off and the result is very powerful, but with unexpected consequences when, due to the speeds attained, the Enterprise is sent back in time by 71 hours. This is the first recorded instance of a Federation vessel achieving time travel.