The Practical Joker
While avoiding a Romulan ambush, the Enterprise passes through an energy field that affects the ship's systems. Soon practical jokes occur all over the ship, and with no officer to blame.
The ship's computer goes insane! That's a new one, at least for Star Trek. It is a gentler soul than HAL, and at no point sings, but does laugh incessantly. It's nice for continuity that the voice is clearly Majel Barrett, who does the computer voice work in all Star Trek series. Also foreshadowing later series is the "recreation room," which is very much like a holodeck, a device especially exploited in The Next Generation. Here it cna only imitate natural environments. Returning to the jokes, they generally aren't very funny, except perhaps the "Kirk is a Jerk" t-shirt. (Though Jim's "Go Climb a Rock" shirt in The Final Frontier is even better, because he chooses to wear it.) The problem and solution are not given a lot of time, but at least they make sense. This is one of the few episodes for which we think "kids would like this," not because its beneath adults, but because of the humour and pace.
Spock Saves the Day: Our Vulcan figures out why the ship is in such a cheeky mood and knows how to reverse the effect.
Oops: Spock falls for the ol' telescope-rimmed-with-ink trick.
Developments: Spock keeps his wits and explains that "Vulcans never laugh," and is generally unaffected by this incident.