The Time Machine in which D&D traveled back in time.
In this Parallel Universe crossover episode, Dutch and Dillon meet Doctor Emmet Brown (Christopher Lloyd), and due to a misunderstanding involving plutonium bars stolen from a Lybian terrorist group, and the accidental death of Brown's teenage assistant, Dutch and Dillon end up travelling back in time to 1955, where they both stop a drug traficking ring set up as a High School Ball, destroying the High School in the process.
The episode ends with a ray of lightning hitting the time machine, which results with Dutch and Dillon traveling back in time once again, this time to the old west. This storyline is continued in what is called by fans the "Dillons Trilogy".