The Doctor takes Adric and a young air hostess named Tegan Jovanka, who has come aboard the TARDIS by accident, to the planet Logopolis, home of a race of mathematicians whose help he hopes to enlist in reconfiguring the outer shell of the TARDIS. The mysterious, wraith-like Watcher brings Nyssa from Traken to join them and warns of impending danger - something that is borne out as the Master arrives and kills a number of the Logopolitans.

The fourth segment of the Key to Time embroils the Doctor (Tom Baker) in a swashbuckling adventure of mistaken identity and intrigue. Romana: Mary Tamm. Count Grendel: Peter Jeffrey. Prince Reynart: Neville Jason. Zadek: Simon Lack. Voice of K9: John Leeson.

The Doctor arrives in present day England to meet a professor experimenting on the fossilized skull of a long dead god-like being. When it awakens and kills, the Doctor jumps into action to prevent others from exploiting the skull's dreadful power.

The travellers land on the planet Marinus where they meet Arbitan, keeper of the Conscience of Marinus, a device that's eliminated evil from the minds of the planet's inhabitants.

On a holiday in Paris, the Doctor (Tom Baker) and Romana uncover a plot to steal the Mona Lisa. Julian Glover. Duggan: Tom Chadbon. Countess: Catherine Schell. Kerensky: David Graham.

Dr. Who is menaced by monsters and beasts. Dr. Who: Jon Pertwee. Jo: Katy Manning.

Dr. Who's assailant springs a surprise. Dr. Who: Jon Pertwee. Jo: Katy Manning.

The travellers land on the planet Marinus where they meet Arbitan, keeper of the Conscience of Marinus, a device that's eliminated evil from the minds of the planet's inhabitants.

Freddie needs a new coat for a "Doctor Who" fan-club event; Ash asks Freddie and Stuart for advice on winning back his ex-girlfriend; and Ash, Violet and Penelope make a shocking discovery during a shopping excursion.

Following a regeneration process, the youthful, new Doctor (Peter Davison) is quickly abducted by a group of warriors. Nyssa: Sarah Sutton. Portreeve: Neil Toynay. Tegan: Janet Fielding.

Part 1. Sontaran warriors set their sights on the Earth and its inhabitants.

Part 2. As poison gas suffocates the Earth's population, the Doctor and UNIT team members work to eliminate the threat.

The Doctor meets the most important woman of his life.

Agatha Christie's 10-day disappearance in 1926 may have been the result of amnesia, a nervous breakdown, or a giant alien wasp.

Part 1. The Doctor and Donna must uncover the truth behind the Nodes and the Data Ghost to uncover the library's secret.

Part 2 . As the shadows rise, the Doctor forges an alliance with the mysterious River Song.