DeeDeemensional is the first episode in season one of Dexter's Laboratory, which first aired on April 28, 1996. In this episode, Dexter has Dee Dee go back in time to tell himself not to open a passage. However, Past Dexter refuses to accept the message, and chaos ensues when Future Dee Dee meets Past Dee Dee.
The episode name is a combination of the word "dimensional" and Dee Dee.
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.