A newly regenerated Doctor joins UNIT in an investigation into the theft of top secret plans and equipment from supposedly secure premises. Sarah discovers that the raids have been carried out by a robot invented by scientist Professor Kettlewell while he was working for Think Tank, a body involved in developing emerging technologies.

Susan learns the Thals are starving.

The Daleks agree to help the Thals.

When the TARDIS lands in the swamp lands, the monstrous Kroll, who swallowed the fifth segment centuries before, rises from the depths. In order to revert Kroll to its natural form, the Doctor must get the key tracer near him before it's too late.

In the Crimean War, the Doctor discovers the British army fighting a brutal alien army of Sontarans, as Yaz and Dan are thrown deeper into a battle for survival.

The Doctor and her friends find themselves in the Deep South of America when they arrive in Montgomery, Ala., in 1955; as they encounter a seamstress by the name of Rosa Parks, they begin to wonder if someone is attempting to change history.

The Doctors alarmed seeing coverage of an archaeological dig into an ancient barrow near the village of Devil's End.

An alien warlord sets a twisted plan in motion in order to ensure his survival on a remote Army outpost, and the Doctor may have to do something unthinkable.

Returning to Gallifrey, the Doctor faces the Time Lords in a struggle that takes him to the end of time itself.