The Doctor robs the Daleks, gains two companions, loses three, and then we even get an appearance by the monk. Epic!
(NB: As this is an almost entirely lost Doctor Who serial, this is another solo-cast.)
The Doctor, Katarina and Steven arrive on the planet Kembel, last seen in the Dalek Cutaway, Mission to the Unknown. Steve’s in a bad way following his intermezzo with a Trojan warrior in The Myth Makers, but perhaps the two new interim companions, Bret Vyon (played by Nicholas Courtney of Brigadier Lethbridge-Stewart fame) and Sara Kingdom (played by Jean Marsh) can be of service.
It transpires that they can help each other, because as we found out in Mission to the Unknown, the Daleks have assembled their allies on Kembel to plan the invasion and/or destruction of the (our?) solar system and/or galaxy and/or universe, and the Doctor is quick to intervene.
The final cog of the titular Master Plan comes in the form of Mavic Chen, the Guardian of the Solar System, who delivers to the Daleks a core of solid Taranium, as a power source for their newly constructed Time Destructor. So when The Doctor steals the core and speeds off with his companions, a race across time and space with The Daleks and Mavic Chen in hot pursuit ensues.
A massive 12-episode epic, not even counting the Dalek Cutaway prequel, this is the longest Doctor Who serial to date, and one of many firsts.