The Daleks' Master Plan

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:

The TARDIS materialises on Kembel. There the Doctor and his friends meet Space Security Service agent Bret Vyon, who has been sent in search of Marc Cory. Learning of the Daleks' scheme, which hinges on the use of a weapon called the time destructor, they determine to warn the authorities on Earth. This proves problematic as the human leader Mavic Chen, Guardian of the Solar System, is a traitor in league with the Daleks.

Fortunately the Doctor has managed to make off with the taranium core of the time destructor, disrupting the Daleks' plans. In one of a number of attempts to regain the taranium, Chen dispatches Space Security Service agent Sara Kingdom to track down the Doctor's party.

Sara, unaware of Chen's treachery, kills Bret - her own brother - before the time travellers can convince her of the truth. She then joins forces with them and, after an encounter with their old adversary the Monk, they all arrive back on Kembel. There, the Doctor contrives to steal the time destructor and turn it against the Daleks, annihilating them. Sara has ignored his instruction to return to the TARDIS, however, and she too is killed.

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