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SPOILERS AHEAD AND IN THE LINKS POSTED HERE
There's a moment in "The Waters Of Mars" where we see The Tenth Doctor doing his ... well, his Thing: working another last-ditch plan that he knows is going to save everyone, and you know it's going to save everyone, and so does the person he's rescuing.
But what makes this episode work is that, even as we see the Doc at his most life-affirming, his most ambitious moment yet, both you and the rescuee - in this case, the formidable Capt. Adelaide Brooke - know that he is absolutely wrong.
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Once again, The Doctor isn't there to save anyone, merely to bear witness. And this time, the Doctor - a truly Lonely God, without a Companion to buoy him or his fellow Time Lords to rein him in - cracks. Seemingly on the side of good, initially, as he gets Adelaide and some of her staff out of harm's way, but the slope is already slippery; as she chides him for breaking his own rules, his only response is a raised eyebrow and one word: "Tough."
If all this sounds like it's ignoring the baddie of the week or the rest of Adelaide's crook, well, it kinda is. They're not bad, as "mid-season" episode groups of characters go - the unnamed lifeform that picks off the crew is clever enough to up the ante as need be - but the episode turns on the final 10 minutes, when Ten goes off the rails and realizes it too late. The Cloister Bell is tolling for him. And this time, both he and you know the end is coming.
Top image courtesy of oneGemini Studios