After the ship traverses an odd cloud, the Enterprise's computer becomes a practical joker.
This episode provides examples of:
- Briar Patching: Passing through an energy field causes the Enterprise computer to play jokes on the crew. Captain Kirk pretends to be scared of the field and tricks the deranged computer into taking the ship through the field again, which reverses the effect that made the computer go bonkers.
- "Everybody Laughs" Ending: Three Romulan ships follow the Enterprise into the energy field, causing their computers to suffer the prankster malfunction even as the Enterprise's is fixed. Kirk promises to tell them how to fix the problem.Kirk: But later. Let's not spoil their fun just yet. [bridge crew shares a hearty laugh]
- Eyepiece Prank: The computer places a viewing device at Spock's console, coating the rims of the eyepieces with a black pigment. When Spock looks into it, it leaves rings around his eyes.
- Gravity Screw: The computer turns off the ship's Artificial Gravity system to stop Scotty from shutting it off, causing the characters to float around in zero gravity until Scotty leaves the computer mainframe room.
- Holodeck Malfunction: The episode gives the original NCC-1701 Enterprise's rec room a hologram feature, a full century and then some before TNG's holodeck. McCoy, Uhura and Sulu are trapped in it.
- "Kick Me" Prank: The computer prints "KIRK IS A JERK" on the back of his uniform tunic.
- Laughing Mad: The Enterprise computer does this in abundance after passing through the energy field, and then causes the crew to briefly do the same by dosing them all with gaseous nitrous oxide.
- Negative Space Wedgie: This one causes shipboard computers to become pranksters for some reason.
- Now Do It Again, Backwards: After the Enterprise passes through an energy field, the ship's computer starts pulling jokes on the crew. The computer is returned to normal when the Enterprise passes through the cloud again.
- Tempting Fate: While chilling in the rec room with Uhura and Sulu, Bones says "Best of all, no practical jokes." A minute later, they walk into a Pit Trap generated by the computer.
- Vengeful Vending Machine: When Scotty tries to obtain a sandwich from the food replicator, the malfunctioning computer sprays food all over him, concluding with a custard Pie in the Face.
