Initiating a final civil war on Skaro, the Dalek Prime has all the Davros loyalists revealed and exterminated. In the meantime he releases the Doctor to leave Skaro.
In a desperate attempt to ensure his own survival, the Dalek Prime identifies and eliminates all those who still support Davros on Skaro. As part of this strategy, he also chooses to release the Doctor, allowing him to leave the planet.