In Innocence, Rory Jennings becomes the fourth actor to portray Davros, but at the start rather than end of his career, as a callous little budding megalomaniac scientist - we completely understand how the youth becomes the Davros we know. An excellent depiction of a troubled family background in an intricate and violent political situation; of all the stories, this shows the most obvious homage to Robert Graves, and that's not a bad thing.
Terry Molloy comes back in Purity, to tell the story of young Davros' entry to the military elite and the continuation of the story of his calculating mother, Calcula. Lots of glorious references to canonical Skaro lore, including not just the Mutos from Genesis of the Daleks but also the Varga plants from Mission to the Unknown/The Daleks' Master Plan; and the political leader of the Kaleds is the Kaled Supremo, a very nice touch. The plot is perhaps the least original of the four stories - Davros and friends sent on a mission which is fore-ordained to failure - but it's very enjoyable.
Corruption reprises quite a lot of the material from the earlier BF Davros play, but in my view (and I may be in a minority) does it much better, going through the court politics around Davros's increasing hold over the Kaled Supremo, and of course what he finally does to his mother, a crucial bit of psychology, at the end of which he adopts her "children", the Kaled Youth movement, at the same time as preparing the inevitable future for the Kaleds' children as a whole.
Guilt is the only true prequel of the four stories, in that it deliberately takes us just up to the point before Genesis of the Daleks, with Davros overcoming disability and captivity to join forces with Nyder (Peter Miles brilliantly reprising the role) and develop the elements which are to be united in the form of the Daleks. Even though the trajectory is pretty much pre-set, it is again an excellent ride.
In summary, a brilliant set of four plays, which I suspect would stand on their own as dramas even for a non-Who fan.