Mock 5 is the third part of the 9th episode in season 2 of Dexter's Laboratory. In this episode, Dexter competes in the big race down Volcano Mountain while trying to beat Mandark.
Dexter enters a soap box derby (initially against his father's will because Dee Dee supposedly "died" from crashing into a turtle, but then he changes his mind on the condition he does it in memory of Dee Dee) but Mandark is going up against him.
"I'm Dee Dee, Dexter's older sister, who two years ago crashed up in the Volcano Mountain Race and was never heard from again! Except I was heard from again, it's just that my Dad likes to exaggerate and he got mad at me because I broke his car and he wouldn't let me race again! Except I had to race again, it was in my plans, so sometimes I stow away in my brother's trunk so I can be close to the action and sometimes I secretly race under the disguise of Racer D, but I shouldn't have told you that!...(long pause) I've got a monkey!"
"Secretly, unknown to Dexter, a mysterious stranger is mysteriously watching..."
In this episode, Dee Dee and Mandark do not seem to recognize each other despite having previously met and Mandark himself shows no attraction toward Dee Dee, instead being in love with her alter ego Racer D.
This likely implies that this episode takes place in a different continuity or that Dee Dee simply did not recognize Mandark due to her usual idiocy and he did not recognize her due to her childish appearance and her explanation being too fast to understand as he was unable to tell she was Racer D despite her unintentionally telling him directly.
Another possibility is that the reintroduction was merely used as an exposition parodying the long and convoluted introductions and explanations often seen in Japanese animated media.
Dad's role and his outfit in this episode is based on Speed's father, Pops and is a reference to the character himself in the series.
The way Dad speaks in this episode is much like Pops who is a fast talker when he had a conversation with his son Speed as is to Dad's son, Dexter.
Dee Dee's role as Racer D in this episode is based on Racer X and she serves as a female counterpart of the eponymous character but changes from X to D.
The turtle that causes both Dee Dee and Mandark to crash in their respective races may be a reference to the Turtle Shell power up used in Nintendo's Mario Kart franchise.