I'm from out of town, the Fifth Dimension to be exact. Name's Mxyzptlk, Mr. Mxyzptlk. It's spelt the usual way.—Mr. Mxyzptlk, to Clark Kent, 'Twas the Night Before Mxymas
Mr. Mxyzptlk (pronounced Mix-Yez-Pit-Leck) was a villain in the series.
He was a Imp from the Fifth Dimension who wanted to take over the Third Dimension where Superman lived and would not let anyone send him back home or stand in his way.
History
Mxyzptlk was an Imp from the Fifth Dimension (which he described as an awful place) who always escaped to the Third Dimension by an inter-dimensional vortex to cause trouble. He would "pull some pranks and have some fun", he even admitted that he caused the fall of Rome.
But someone always found a way to send him back to his own dimension by tricking him into saying his name backwards "Kltpzyxm".
Season 4
Mxyzptlk decided once and for all that he was going to stay in the Third Dimension and no one would send him back. He created another inter-dimension vortex to send him into Metropolis. The process required a great deal of energy and entering the vortex gave him a brain freeze.
He knew Superman was the only one who could stop him from taking full control so he came up with an ingenious scheme to hold the Man of Steel at bay: if there was no hope in the world, Superman who was the symbol of hope would be powerless.
First he observed Superman in action and then encountered the Man of Steel just as he was heading back to his workplace the Daily Planet.
Mxyzptlk starting the time loop
The Imp introduced himself, saying he knew Superman's identity but it didn't matter to him. He then activated a time loop that made the day repeat itself whenever the clock reached four. Each time people's attitudes towards each other changed including Clark's wife Lois Lane and both their parents. Even the newsroom Christmas Tree got smaller and smaller the more the loop continued.
Mxyzptlk arrived and Clark demanded to know what he had done. The Imp replied that he had taken away tomorrow to get rid of hope to prove a point that he was there to stay and nobody was making him leave this time. He also left Clark out of the loop for a reason to show that there was nothing he could do to stop him.
Clark tried to grab him, but Mxyzptlk escaped, leaving behind a bouquet of flowers telling the Man of Steel that he couldn't hurt him and would return if he was ready to make a deal.
The loop went on with Clark's father Jonathan dying from loss of hope. Mxyzptlk appeared again with an angry Clark demanding him to set them free or else he would send him somewhere far worse than his Fifth Dimension.
Mxyzptlk making a deal with Clark
The Imp replied that it was just the affects of the loop running it's course with Clark's father being the first to die. After Clark agreed to make a deal with him, Mxyzptlk gave his terms: he would end the loop and give the people back their hope if Clark agreed to leave Earth forever so that the Imp would rule it all.
After thinking it over, Clark refused and his hope broke Lois free from the loop as well. Mxyzptlk was furious and decided to continue the loop until Clark changed his mind. "Fine, have it your way!"
He appeared one last time at the Daily Planet and was astonished to see everyone suddenly looking so cheerful. Lois and Clark had been working together to prevent their family and friends from losing hope which left the Imp astonished and annoyed as they were not supposed to be delighted.
He confronted Clark who was now dressed as Superman that all this "love and good cheer" would never break the loop and he would still be supreme ruler.
Mxyzptlk disappearing to his dimension after being tricked.
Suddenly Lois appears with a present. Daily Planet editor Perry White dressed as Santa Claus called out his name. The Imp was taken aback as no one had ever given him a present before. He excitedly took it from Perry and showed great excitement as he has no idea what it could be.
Lois then urged him to read the card to see who it was from. He then read the card attached to it that said: "To our beloved Mr. Mxyzptlk", to which he felt extremely touched. He then read who it was from: "Kltpzyxm".
Suddenly to his horror, he realized that he had just said his own name backwards. But it was too late and he disappeared back to his own dimension. After he left, the loop was broken forever and everyone's hope was restored.