A malfunction on the Enterprise leads to playful, dangerous tricks by the ship itself.
The Enterprise encounters a strange cloud that affects the ship's computer, and soon the vessel begins playing tricks on the crew, some of which are dangerous. The crew must find the reason for the odd behaviour, and Kirk must get control of the ship back.
The story is notable for the first appearance of the holodeck, which in this episode is a recreation room that creates scenes for the crew. It is often cited as an early version of the idea that was later seen in The Next Generation, and it is a favourite of fans for that reason.