Spock is stricken ill with a disease that is fatal to Vulcans. With time running out, the U.S.S.Enterprise must rendezvous with the S.S. Huron, a freighter carrying the only known antidote. However, Orion pirates hijack the shipment and Kirk must deal with them before Spock dies.

After avoiding a Romulan ambush by three of their warships, the U.S.S. Enterprise enters a strange energy cloud. The gaseous cloud affects the ship's computer turning it insane. With the crew at the mercy of the Enterprise's computer, it indulges in a series of increasingly aggressive practical jokes.