Turlough's attempt to destroy the Tardis causes the ship to attach itself to a nearby space liner.

The ship's passengers are discovered to be space lepers being transported to Terminus for radiation therapy.

Nyssa contracts the dreaded space leprosy.

The Doctor enlists the aid of the Garm in averting the Second Big Bang.

The Doctor, Sarah and Harry return to the ark to recover the TARDIS but find themselves in a different time with a crew incapacitated by a mysterious, deadly plague. With Tom Baker.

Aliens offer Earth a gift.

A seemingly friendly alien spaceship lands on Earth.

The Doctor discovers that the alien spacecraft and all on board are one organism.

The distribution of Axonite around the world threatens to draw off all Earth's energy.

The Brigadier insists upon exploring the caves.

A Silurian stuns Liz, murders a scientist, then sets his sights on the Doctor.

When the Doctor tries to make peace with the Silurians, they kidnap him.

The Silurians inflict a plague on humanity.

The Doctor seeks an antidote to the plague unleashed by the Silurians.

The Silurians plan to destroy the Van Allen Belt in their plot to reclaim Earth.

The Doctor investigates alien involvement in the mysterious disappearance of spacecraft sent from Earth to Mars.

The rescue ship returns to Earth, but when the hatch is forced open, it is empty.

The Doctor learns that the astronauts are really Martians.

The Doctor travels to Mars to find the Earth astronauts.