Passing through an energy cloud has a strange effect on the USS Enterprise. The computer (voice by Majel Barrett) begins playing pranks on the crew, which has to also deal with the Romulans.
From season 2 of the animated Star Trek, here's "Practical Joker", or at least 5 minutes of it, courtesy of CBS,which owns the rights.
Star Trek: The Animated Series airs Sunday nights with 2 back-to-back episodes to kick off Heroes & Icons' block of all six Trek series (the 5 live action series run Sunday-Friday from 8 pm-1 am ET). Check your cable system to see if you have H & I.