When a Thal platoon arrive on a hostile planet investigating reports that Dalek artefacts have been detected, they are unprepared for what they find. In an underground room is a stranger, a Professor, or so he claims, with no memory of who he is or why he is there. With death and horror their only companions, the Thals make their way with the Professor into the heart of a crumbling Dalek citadel in search of answers... only to find that the Daleks are the least of the horrors they must face.
The Dalek Emperor is overseeing the implanting of a "Dalek heart" into different lifeforms in order to create a new, more powerful Dalek race so that only Daleks remain. The Emperor would try to reconstitute the Dalek race by transforming human into Daleks in TV: The Parting of the Ways.
Part of the Dalek experiment is to create sleeper agents whose outward appearance matches their home planet, but who have the mind of a Dalek. This appears to be an evolutionary middle step between Robomen and the Dalek puppet. Dalek puppet sleeper agents would appear in TV: Asylum of the Daleks and TV: The Time of the Doctor.
Because of the ubiquity of this race, no attempt will be made here to list every appearance in every medium. Instead, we focus on prose appearances here, and invite you to explore a complete list of appearances elsewhere.