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I think I've sussed what's going on in the final series of Ashes To Ashes. There were certain things I spotted during the second episode that could be carefully laid clues. Then again I might be totally wrong. I'll list my ideas here and we'll see how close I was at the end of the series.
I think Jim Keats is the Devil. The reasons for this:-
(1) His office was in the basement - A metaphor for Hell.
(2) His office was scorching hot - As hot as Hell.
(3) Someone commented his office was fetid, meaning rotten or decaying.
(4) He wears horn-rimmed glasses - the Devil's horns.
Jim is also using his charm to try and get Alex on to his side, telling her he can help her, something the Devil would do. And does he really know where she is from?
If you remember in the first series Gene Hunt saved Alex as a child from the exploding car, so it's a good possibility that he is her guardian angel, which is why he gets up Jim Keats' nose so much. That would mean that the world they inhabit is purgatory, a halfway house between life and death, Heaven and Hell.
Again this is only a theory of mine and I could have got it all horribly wrong, but as I said let's see at the end of the series.
And as for Sam Tyler, I think he's very much alive and Gene Hunt is covering up for his friend (acting the guardian angel), but why? In Life On Mars Sam Tyler discovered his father was a crook and I think the bank robbery that supposedly got Sam killed was carried out by his father and Sam let him escape once again, but this time he followed his father to get to know him better. Well, that idea is a little dodgier than the Devil/guardian angel one above, but again we'll see.
What do you think?