Doctor Who is a show that has given us a cultural heritage of hiding behind sofas, a shared experience of fear. So when we say that The Waters of Mars is hands down the scariest, most gruesome episode of Doctor Who, that's saying quite a lot. There's something especially terrifying about a mysterious virus infecting people at will and dooming them to death, probably because it hits a little too close to home. We know that the odds of us being attacked by a Dalek are relatively low, but death from disease is much more within our sphere of experience. The growing terror of being trapped in an enclosed space with a killer virus is remarkably compelling, and the resulting tension is palpable. But as scary as the Mars monster is, that's nothing compared to how frightening it is to see the Doctor try to play God. He's completely unhinged and out of control at the end of the episode, and his manic outburst comes totally out of nowhere.