Let’s start with the obvious thing:

I have a story in which Davros breeds the Daleks on Earth from humans. So the question is, how should the Daleks leave the Earth without exterminating the entire human population?

One idea I had is that the Sontarans come and drive the Daleks off, because it turned out Davros got help from the Rutans.

Another is that a bacteria they have no immunity to is placed in their feed, and thus a bunch of Daleks die from an infection they have no immunity to because they have no immune systems.

Or Movellans!

Or the Daleks already had plans to leave the Earth!

Or some comination of these things!

What do you think?