Serial: QQQ  Episode Nos. 338, 339, 340, 341, 342, and 343.  Title: Frontier in Space

In the 26th century, humanity has spread into the galaxy and built an empire rivalled only by that of the noble Draconians.  A cold war between the two sides is rapidly heating up as each believes the other to be the aggressor in a series of attacks on their spaceships.  When the Doctor and Jo land in the middle of one such attack, both sides presume them to be involved.
The Doctor and Jo have seen the truth, however, that the attacks are being perpetrated by the Ogrons under orders from the Doctor's archenemy, the Master...

starring Jon Pertwee as Dr. Who, Roger Delgado as the Master, and Katy Manning as Jo Grant. Written by Malcolm Hulke, Produced by Barry Letts, and Directed by Paul Bernard.

Originally transmitted from 24 February 1973 to 31 March 1973 on BBC1.