Andy Stone Infected by The Flood

Arriving on Mars, The Doctor steps out of his TARDIS and admires the Martian landscape. As he explores he spots, in the middle of a large crater, a large base. As he looks down on it a remote-controlled robot detains him and The Doctor finds himself escorted to the base.

Inside he discovers the base, called Bowie Base One, is inhabited by a team from Earth. It is in fact the first human outpost on Mars and that the date is the 21st November 2059. On learning this The Doctor realises that he must leave as staying could end up in him unwilling breaking the laws of time and so interfere with a fixed point in history. The Doctor knows that on this date Bowie Base One was destroyed in a mysterious explosion and that all the crew were killed.

However, as The Doctor starts to leave a crisis develops in the bio-sphere section of the base. Two crewmembers, Andy Stone and Maggie Cain, have been infected by a mysterious life form (known as The Flood) which takes over their bodies and causes them to gush copious amounts of water. The base commander, Adelaide Brooke, believes that The Doctor could be responsible for the infection in some way, and orders him to come with her and another crewmember, Tarak Ital, to investigate.

Tarak Becoming Infected

On their way there they discover Maggie Cain unconscious on the floor. While Maggie is taken to the medical wing, The Doctor, Adelaide and Tarak enter the bio-sphere. As they split up to look around, The Doctor and Adelaide discover Andy Stone but they soon realise that he has become infected and that he has passed on his condition to Tarak.

The Doctor and Adelaide, realising there is nothing they can do to help either crewmembers, rush back to the central control centre. They are chased by both infected men and they only just reach the air and water-tight part of the base in time. On entering the central control centre The Doctor and Adelaide learn that Maggie Cain, who is secured in the medical wing, is also infected and that the organism occupying Maggie's body has revealed its desire to reach Earth, a planet rich in water.

Adelaide Brooke

On questioning the crew The Doctor learns that the base has been built on top of a large underground glacier which is being used as the base’s supply of water. On further investigation it is discovered that the filtering system has malfunctioned so allowing infected water, which had been locked up in the ice, to enter the base’s water system.

Adelaide then learns from The Doctor the fate of the base and that they are all going to die, despite the fact that The Doctor also reveals that her death will inspire her descendants to travel further into space and establish peaceful relations with numerous extraterrestrial species. With this startling revelation Adelaide orders the evacuation of the base and the preparation of the escape shuttle. As The Doctor makes his way back to his TARDIS, he leaves the crew of Bowie Base One to collect essential supplies for their trip back to Earth. However, they are initially unaware that the infected crew members from the bio-sphere have gained access to roof of the control centre and are exuding more water. It is not long before the water finds its way inside the control centre and starts to pour from the ceilings.

Maggie, having been left alone, then breaks out of her confinement in the medical wing. She infiltrates the shuttle and sprays water over Adelaide's deputy, Pilot Ed Gold, as he prepares the shuttle for take-off. Realising that he has been infected, and before the condition takes a hold over him, Ed manages to trigger the shuttle's self-destruct mechanism, which traps the infection on Mars but also leaves the surviving crew with no means of escape.

Water Everywhere

The destruction of the shuttle is witnessed by The Doctor who, overcome by defiance against time itself, decides to try and save the others. However, on his return to the base he discovers that Adelaide has activated the base's self-destruct sequence. In a race against time The Doctor uses the remote-controlled robot to access the TARDIS, operate its controls remotely and transport his time machine into the base – so enabling him to rescue Adelaide, and two of her remaining crew, Yuri and Mia Bennett, from the resulting nuclear explosion.

The TARDIS then materialises outside Adelaide's house on Earth on exactly the same date. Mia and Yuri, shocked by their experiences on Mars, quickly depart leaving The Doctor and Adelaide on their own. As Adelaide starts to quiz The Doctor about his actions, and why he ultimately decided to save her and the others, The Doctor announces that he has come to realise that the laws of the Time Lords were only valid while their civilisation existed, and that since he is the last of his race he has total authority over time. Scolding The Doctor for his newfound arrogance, Adelaide returns home and commits suicide, meaning that The Doctor's efforts at changing the timeline are mostly undone, the only differences being the actual location of Adelaide's death and Yuri and Mia's survival.

Only now understanding the full impact of his actions, The Doctor is overcome with horror and realises that there will be a price to pay for his interference. He then spots Ood Sigma further up the street, prompting The Doctor to wonder if this time he has finally gone too far and that whether the time has come for him to die. Sigma then vanishes and as The Doctor staggers back into the TARDIS the ominous sound of the Cloister Bell can be heard…