A group of cult members do a black magic ritual to unleash the true power of whatever and accidentally bring Mr. Mxyzptlk to Earth. Meanwhile, Young Justice members Superboy, Robin and Impulse host a hallow-teen party for teenagers around the neighborhood.
Robin gets tangled with a Jock trying to impress his girlfriend while Impulse and Superboy deal with a crisis: who turned all the kids into elders and why can't they stop dancing?
Mr. Mxyzptlk then appears and tells them he is doing research.
As Young Justice pounds him to make him turn things back to normal, Mr. Mxyzptlk explains he was going to turn things back to normal after his research. Superboy is shocked that this version of Mxyzptlk is a scholar, while Robin warns him NOT to tell him about his future self telling him it may affect the present.
But Superboy tells Mxyzptlk that he rots Superman's life with his stupid occasionnal pranks.
The present is thus destroyed and Young Justice have to fix it all.
They bring Mxyzptlk to the cinema, to watch a comedy movie, but instead of laughing to the movie, he laughs at the projector kid, Mick Gurk. After giving an electric handshake to Impulse, laughing at everyone and thanking Young Justice for showing him humor, he disappears.
They then go out and look around them, as they see that Mxyzptlk has had fun and turned to whole area to a "Mxyzptlk Halloween", while Robin asks Red Tornado if the whole world looked like this or just the immediate area. Luckily for them, it was only the immediate area, and Robin the wonders if Mxyzptlk and Impulse were seperated at birth.
The bullies that try to rob candy from Red Tornadoes daughter, Traya Sutton, resemble Jimbo, Dolph, and Kearney from the Fox TV animated show, The Simpsons (1989-Present).