Two young English women go on a cycling tour of the French countryside. When one of them goes missing, the other begins to search for her. But who can she trust?

Intercepted while travelling between Earth and the Ark, the Fourth Doctor and his companions are transported to the planet Skaro on a mission for the Time Lords — to prevent the creation of the Daleks.

The TARDIS materialises in a hostile jungle on the planet Spiridon. Jo sets out alone to find help for the Doctor, who has fallen into a coma. She meets a party of Thals and is left in hiding aboard their crashed spaceship while they go to the Doctor's aid. The Time Lord, now recovered, learns of their mission to destroy a party of Daleks sent here to discover the native Spiridons' secret of invisibility.
Another Thal spaceship crash-lands in the jungle, and the survivors bring news that somewhere on Spiridon there is an army of ten thousand Daleks. Jo meanwhile meets a friendly Spiridon named Wester, who cures a deadly fungus disease that she has contracted.
It transpires that the Daleks' army is frozen in suspended animation in a cavern below their base. The Doctor, with the help of the Thals, explodes a bomb in the cavern wall and thereby causes one of the planet's natural ice volcanoes to erupt, entombing the army in a torrent of liquid ice.
The newly-arrived Dalek Supreme and his aides are left stranded on Spiridon as the Thals steal their ship and the Doctor and Jo depart in the TARDIS.

维基：

The Doctor and Sarah reach Earth and try to warn the research centre about the Kraal invasion before Styggron unleashes his virus.

It consisted of five stories, plus the incomplete Shada, unfinished as a result of an industrial strike. Lalla Ward joined as Romana II, and the season is notable for the involvement of Douglas Adams in the writing department. This was the last season to use the traditional Delia Derbyshire "Doctor Who theme" arrangement.

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