Of all the monsters that the Doctor has encountered in Doctor Who, the Flood creatures from the 2009 TV special "The Waters of Mars" were some of the scariest, but they also required explanation. In lieu of a complete season, a series of extended Doctor Who specials was produced between late-2008 and early-2010 that saw the exciting final adventures of the Tenth Doctor. Across the five-episode run, the Doctor encountered classic foes like Cybermen, but also battled an original monster in the form of The Flood. With the specials culminating in his regeneration, the Doctor's brief encounter with The Flood still tied into the overarching theme of his last days.
Besides being an excellent monster-of-the-week story, "The Waters of Mars" also saw the Time Lord struggle with one of his darkest moments, and illustrated that adventures in time and space weren't always happy. ed as a great Doctor Who episode set in the future, "The Waters of Mars" also explored the idea of a fixed point in time, and how the Doctor was unable to change it. Faced with a dark foe in the form of The Flood, the Doctor went against his own moral code when trying to alter the future, but paid the ultimate cost by becoming directly responsible for the persistence of time.
The Flood Was A Virus That Possessed People Through Water
"The Waters of Mars" was set in the year 2059 on Mars colony Bowie Base One, and showed the Doctor interacting with some of the first human settlers on the planet. For water, the base drew from a nearby glacier which just happened to be the ice where an ancient being known as The Flood had been trapped for some time. Reminiscent of 1980s horror/sci-fi films like 1982's The Thing, The Flood infected those who came in with the water which turned them into the living dead. Complete with a ghoulish new visage, an infected individual began leaking water from every pore and was host to the creature.
The possession granted the victim keen senses and more agility than the average human, and they were also invulnerable to most attacks and could survive on the surface of Mars. The Flood themselves could control the movement of the water that they lived in, which made the liquid itself a dangerous enemy for the surviving colonists. Besides being a frightening visual for Doctor Who's best monster, the purpose of the water spouting was to turn them into walking hosts who could spread The Flood to others with just a single drop of their water. Infection from The Flood was permanent, and no one was shown to have survived it.
Why The Flood Possessed The Crew Of Bowie Base One
Being a microscopic viral organism, The Flood was like most viruses and needed a host body in order to thrive and grow. Ironically, as the denizens of Bowie Base One were building their home on the inhospitable planet of Mars, The Flood was looking for an equally comfortable home within the human hosts. Somewhat powerless in their singular form trapped in the ice, the use of a host body made them more mobile, and they could spread their infection wider. The best Doctor Who special episodes often introduced novel concepts into the universe of the show, and The Flood was a one-off villain unlike any that had been encountered before.
How The Doctor Managed To Defeat The Flood
Unlike most Doctor Who adventures which saw the Time Lord win the day with his clever outside-the-box thinking, "The Waters of Mars" saw him limp away and almost lose everything. Earlier in the episode, it was established that the Bowie crew's leader, Captain Adelaide Brooke, would blow up the entire base under mysterious circumstances. With that moment meant to be fixed in time, the Doctor spent the whole episode struggling with the idea that there was nothing he could do to reverse the outcome. In the end, he saved Brooke and a few crew with the TARDIS, while the rocket pilot exploded the ship in order to defeat The Flood.
Though the Tenth Doctor was beloved for his eccentricity and fun-loving personality, "The Waters of Mars" put him through Doctor Who's greatest emotional test when he then had to deal with the consequences of trying to rewrite the future. Captain Brooke learned that she was supposed to die on the base and inspire her granddaughter to be a great astronaut, and opted to take her own life to preserve the timeline. While The Flood was defeated, it wasn't by the Doctor, and his meddling actually made the outcome much darker by placing Brooke's death on his own shoulders.