Action Comics #80

1. Mr. Mxyztplk Returns (Superman) Reprinted in Superman #272 & Superman: The Action Comics Archives #5.
2. The Riddle of the Reservoir (Congo Bill)
3. Grandpa Peters
4. The Pillage of the Parthenon (Vigilante)
5. Casey the Cop
6. Escape by Night
7. The Hair that Wasn't There (Hayfoot Henry)
8. Tumbling Timber (Zatara)

Superman: Clark's day at the Planet suffers from several seemingly minor mishaps - his alarm clock goes off 3 hours late, he makes mistakes in writing articles, and takes Lois to a pet shop for lunch. Just as Clark thinks he's going crazy, the cause of the pranks shows up - Mr Mxyztplk. (Incidentally, the problem of spelling the imp's name seems to have bedeviled the authors of the various Superman comics over the years, as in the original Siegel/Shuster stories, the last 4 letters of his name are 'tplk', while in the later comics, they are 'ptlk'.)

His pranks gradually get more serious, as he invades the Planet's offices, then a cinema, then causes a train to jump off its tracks, and finally causes the Metropolis river to run backwards and flood the city. Fortunately, Superman is able to intervene and fix things before anyone is hurt or killed, but when he finally tracks Mr Mxyztplk down he must still trick him into saying his own name backwards in order to banish him back to his own dimension.

This time, Superman hits on the idea of pretending he wants to say the backwards name himself, but can't remember what it is. When he says something in an attempt at the name, he relies on Mxyztplk's vanity to correct him, and sure enough the trick works.

He also says that he may someday say the magic word and come to Mxyztplk's dimension to plague him in return.