Cruise plays a psychopathic hitman who hijacks a cab driven by nice-guy loser Jamie Foxx (another fine performance) as he carries out five contract killings over one night in LA.
There was an episode of Star Trek where Kirk is split in two, by a 'teleport malfunction' (hem hem) into nice and nasty Kirks. Neither one can function properly, and the task is to reintegrate them. This film works in the same way, neither Cruise nor Foxx are functioning human beings, and as in the Trek episode, the good must take the shadow into itself to become a real person.
My son, who went to see it with me, pointed out that for 95% of the film you can believe that Cruise is just a projection of Foxx's evil side (like Mother in Psycho: 'Oh my god, you killed him, now I'll have to clean it up') and the script carefully ensures that nobody sees the two of them together as separate people. The Cruise character fades away, and people only see Foxx. An exception is when they are talking to Foxx's mother (mother again you see) and there Foxx keeps saying 'she is only talking to you, she's acting like I'm not even here', so it is slightly ambiguous.
Anyway, I can't go through the whole film explaining how I think this works, but suffice to say, without giving any spoilers, that this conceit breaks down at the very end, but only after a certain amount of symbolic shooting at faces in mirrors and breaking through glass. I would like to think this final explosion of the ambiguity was imposed by evil wicked studio executives.