DeeDeemensional

"DeeDeemensional" is the first segment and episode of the first season of Dexter's Laboratory, which first aired on April 28, 1996, although it tends to get mixed up with "Dexter's Rival" because this episode was actually the first episode to air on TV, even though "Dexter's Rival" was the first episode to be produced.

Synopsis
After a monster comes out of an inter-dimensional doorway that Dexter opens, he quickly writes a note that he gives to Dee Dee, instructing her to go back to the past and give him the note before he opens the passage. Dee Dee turns on the time machine and stands next to it, only for Dexter to tell her she needs to be inside the machine. Dexter in the past doesn't believe Dee Dee, thus he has to do whatever she and Past Dee Dee tells him in order to obtain the message from the future.