Classic Doctor Who: The 10 Timey-Wimiest Episodes

This story has the Third Doctor and Jo Grant meeting some time traveling mercenaries from a future where the Daleks reign supreme. They have come to the present day to assassinate a leader that they believe is responsible for their world’s defeat.

In a twist that is now over-familiar but was then more fresh, it’s revealed that their very act of time traveling has set about the events the mercenaries are trying to prevent. The Doctor has to help them to realize this, and to repair the damage to time that they are causing. This was one of the first times that Doctor Who had explicitly dealt with alternate futures, and ultimately involved a whole timeline that never comes to be.

And as a bonus bit of timey-wimeyness, the serial even includes a moment where the Doctor and Jo cross paths with themselves—a few days out of sync with their future / past selves.