Doctor Who: 10 Moments Of Mind-Blowing Unexpected Cruelty From The Doctor

6. "Waters Of Mars"

Among Tennant's most memorable episodes as the Doctor was the first of his three farewell specials. What made the episode so memorable was its uncanny ability to give audiences a genuine chill. With the monsters, the claustrophobic setting and the Doctor's behavior, Russell T. Davies seemed to challenge Moffat's reputation as the "scary" Who writer. In this, the Doctor plays god essentially, as he chooses who is or isn't worthy of survival. He attempts to rewrite what he knows is a fixed point in history by allowing for someone who is supposed to die to live. Though this may border more on cowardly than cruel, it sure made for an unexpected and grim ending.