Doctor Who: The Timey-Wimiest Episodes Not By Steven Moffat

10. The Waters Of Mars (2009)

Prior to this story, the series had introduced he idea of the “fixed points in time”-- moments which simply had to happen, either because of an intrinsic property of time or because of the importance of these moments in history.

So when the Tenth Doctor meets a doomed expedition on Mars, he feels powerless to interfere, knowing how critical this moment is to humanity’s future. But, overwhelmed by the horror of what is happening, he comes to a dangerous epiphany-- as the last of the Time Lords, the decisions about what is or is not permissible are his alone to make.

Acting according to his conscience, the Doctor saves the last members of the team and brings them to earth, not caring about the implications. However, one of the survivors, Adelaide Brooke, realizes the dangers and takes her own life to preserve the course of history, teaching the Doctor an important lesson about his arrogance.