Widely considered one of the best of the specials leading up to his departure, “The Waters of Mars” shows the Tenth Doctor at his most unhinged. Here, he makes the decision to change history and save the lives of Mia, Yuri, and Adelaide Brookes (Lindsay Duncan), despite the fact that their deaths are a fixed point in time. After years of guilt and grief over the death of the rest of the Time Lords, here the Tenth Doctor embraces the power that comes with being the last of them. “The laws of time are mine,” he says with concerning conviction, “and they will obey me!”

Although him successfully saving these people seems like a triumphant moment for a second or two, it quickly becomes clear that the Doctor’s going down a bad path. He dismisses the other deaths on the space station as “little people,” and Adelaide’s disgusted by him. “For a long time now,” the Doctor says, “I thought I was just a survivor, but I’m not. I’m the winner. That’s who I am. The Time Lord Victorious.” It sounds like something the Master would say, not our beloved Doctor.

Adelaide shoots herself soon after, though, and the Tenth Doctor comes crashing down to Earth. The final few minutes see him at his most desperate, realizing both that he’s gone too far and that the time for his regeneration is looming over him. “The Waters of Mars” takes the Tenth Doctor as close to losing sight of himself and his ideals as he’s ever been, and Tennant more than rises to the challenge of depicting his downward spiral.