The Doctor and Peri arrive on the planet Necros, home of Tranquil Repose, a funeral home for the galaxy's elite.

"Inferno is a top-secret project that involves drilling down into the crust of the Earth to unleash a new energy source. However, the Doctor, along with his assistant Liz Shaw, is concerned that this drilling will have disastrous consequences for the whole world. His concerns are soon confirmed when the TARDIS throws him into a dangerous parallel universe, where Project Inferno is almost complete and the end of the world is near..."--Container.

Having left Leela on Gallifrey, the Doctor and K9 Mark II don't remain footloose for long. The TARDIS is soon intercepted by the White Guardian. The White Guardian sets the Doctor on an urgent mission to locate the six segments of the Key to Time, which are well disguised and hidden in odd corners of time and space.

After an encounter with the deadly Mandragora Helix, the Doctor and Sarah Jane land in 15th Century San Martino. In the midst of danger, secrecy and intrigue, they witness the flowering of the Italian Renaissance. However, the Doctor realizes that a third visitor has arrived with him in the TARDIS. It is a force with the power to wipe out human civilization forever. The Doctor has brought it to Earth, and only the Doctor can stop it.

The search for the fifth segment of the Key to Time brings the Doctor and Romana to the swampy terrain of the third moon of the planet Delta Magna. Almost immediately, they become separated.

When the TARDIS lands 50 years after its first visit to Peladon, the Doctor and Sarah Jane discover a troubled planet ruled by the late king's daughter. Peladon is at the center of a war - and the Galactic Federation desperately needs a mineral found in the mines. But why is the ghost of Agreddor killing miners, and why can't everyone be trusted? Queen Thalira needs the Doctor's help to find out.

Taking a test flight in the TARDIS, the Doctor and Jo make an inelegant landing on the planet Peladon. When they are mistaken for representatives from Earth, the Doctor soon finds himself chairing the committee of alien delegates assessing Peladon's petition to join the Galactic Federation. When one of the King's advisors is killed, the High Priest fears the ancient Curse of Aggedor is at work, but the Doctor suspects his old enemies the Ice Warriors...