At the International Gallery, four guards take their places around a priceless artefact. The alarms have been set and all is well for the night. Except that one person has broken in and is able to steal the artefact. However, just as the Lady Christina De Souza is about to make her exit the alarms go off. Outside she spots her getaway driver being restrained by the police. Seeking an alternate means of escape, Christina watches as police cars arrive on the scene. With her options running out, she catches a ride on a bus. A man boards the bus behind her and although there are unoccupied seats next to her, he sits himself beside Christina.

D.I. McMillan looks around him trying to figure out where the perpetrator has disappeared to. He spots her through the window of the bus as it drives past. Seconds later, police cars set off in pursuit, as the bus proceeds along its route which will take it through a tunnel. Sealing both ends of the tunnel, McMillan is confident that he will finally catch his thief. On board the bus, the strange man produces a device that he is using to monitor certain types. As the device's readings become more frantic, the man urges the passengers to hold on...and the bus lurches.

Once the passengers pick themselves up they are amazed to find that they are no longer in a tunnel at night, instead the bus has come to rest on the middle of a vast desert. The three suns in the sky make it clear that they are not even on Earth even more. One of the passengers is worried though. Before the bus entered the tunnel she could hear voices. Now, on this strange planet, she hear mnay more voices...voices of the dead. And something is coming...

Mars: the red planet - the Doctor has a look around and spots a base. A patrolling robot takes him to the base and there the Doctor meets the people who live and work there. His enthusiasm wains when he discovers that the people there are not from some later date - these are the first humans on Mars. Ascertaining the date as 21 November 2059, the Doctor decides that he has to go. For he knows that on 21 November 2059, Bowie Base One and its crew were destroyed - not one of them survived the detonation of its nuclear plant.

Before the Doctor can leave though, two of the base crew have already come into contact with something very sinister indeed...something in the water...something that soon focuses on the water on Earth. The Doctor then realises that not one drop must be allowed back on Earth...and water can be very patient, who can tell how many of the crew have been infected. His choice appears to be simple - leave the base and allow the crew to become a part of history...

The people of Earth are united by a nightmare: the nightmare of a man laughing; the people of Earth are almost united in their inability to recall the nightmare; one human remembers the nightmare - Wilfred Mott. Each and every day he wonders if the Doctor will come and resolve the situation.

The Doctor returns to the Ood-Sphere, having taken a round-about journey through different times and worlds. Ood Sigma is concerned that the Doctor has delayed coming to the Ood-Sphere but brings him before the Elder Ood. The Ood are troubled by the nightmare of a laughing man and when they share the nightmare with the Doctor he ie troubled by the revelation that his old enemy is returning from the dead. Even more troubling is that the events of 21st century Earth are tied to events on present day Ood-Sphere. With each passing second, parallel events take place - the Doctor runs back to the TARDIS and sends the ship hurtling back to 21st century Earth...only to arrive too late. Now things are starting to happen...