Serial: 4B  Episode Nos. 390 and 391.  Title: The Sontaran Experiment

The Doctor and his friends transmat to a supposedly uninhabited Earth, purified by the solar flares 10,000 years previously.
They are therefore quite surprised to find a small group of ragged hostile men, some of whom are subjects in callous tests of human physical limitations being carried out by a Sontaran warrior in preparation for an invasion of the entire galaxy...

starring Tom Baker as Doctor Who, Elisabeth Sladen as Sarah Jane Smith, and Ian Marter as Harry Sullivan. Written by Bob Baker and Dave Martin, Produced by Phillip Hinchcliffe, and Directed by Rodney Bennett.

Originally transmitted from 22 February 1975 to 1 March 1975 on BBC1.
 

Serial: 4E  Episode Nos. 392, 393, 394, 395, 396, and 397..  Title: Genesis of the Daleks

The Doctor and his friends are intercepted by the Time Lords and sent on a mission to avert the creation of the Doctor's most dreaded adversary, the Daleks.  Together they find a world ravaged by centuries of warfare, and from it's ashes, a mad, crippled scientist called Davros seeks to devise a war machine in which the creatures his people will mutate into can survive and conquer the universe...

This story is the first to feature Davros, the creator of the Daleks.

starring Tom Baker as Doctor Who, Elisabeth Sladen as Sarah Jane Smith, and Ian Marter as Harry Sullivan. Written by Terry Nation, Produced by Phillip Hinchcliffe, and Directed by David Maloney.

Originally transmitted from 8 March 1975 to 12 April 1975 on BBC1.