Genesis of the Daleks, Part Four

The planet Skaro, the distant past. The Time Lords divert the Doctor, Sarah and Harry to this war-torn world, and give the Doctor a mission: the evil of the Daleks is about to be born here, and the Doctor is to try to ensure they evolve into less lethally aggressive beings - or, failing that, to avert their creation altogether. But has even the Doctor the right to commit genocide?