The time is the present, the location New Orleans. A massive car bomb destroys a ferry packed with families, and ATF agent Doug Carlin (Denzel Washington) is assigned to the team to investigate it. Be warned: I try to avoid spoilers in my reviews, but it really isn't possible with this one, so if you plan to watch the film and want the plot to be a surprise, stop reading NOW!
Carlin discovers that the FBI are using a "time machine" based on wormhole technology which allows them to look at any point within a radius of a few miles of the machine, but with a fixed timelag of four days and six hours. As the team look back into the past to discover clues to the identity of the bomber, they focus on a young woman, Claire Kuchever (Paula Patton) from whom the bomber obtained the vehicle used in the attack – and whose body had been found in the water near the explosion. Over days of observation, Carlin falls in love with her and, after the bomber has been identified and arrested and the case is closed down, goes back into the past to try to rescue her and to prevent the bombing from happening.
So far so good: it seems to be a straightforward alternate time-line story, with the branch point being Carlin's attempts to alter the past, from which moment the "future" divides into the original time-line in the first part of the film (lets call it TL1) and the new one caused by Carlin's actions, which runs in parallel (TL2). This division into two time-lines is specifically acknowledged in the film, when one of the characters considers the implications of altering the past. Unfortunately, there are some massive plot holes which make a nonsense of the story, which is a shame because it is otherwise an intriguing and entertaining film with some neat touches. If you don't want to know what the problems are, stop reading NOW!
The problem is that the film-makers get terribly confused between the time-lines. This first becomes obvious during a dramatic and original car chase in TL1, in which Carlin is driving a vehicle while trying to follow, through a portable viewer linked to the time machine, the bomber who is driving the same route 4+ days in the past. He is so distracted that he causes a series of accidents, and when he has to double back he drives past some blazing wrecks – but these also appear in the view of the past. That's just carelessness, but a more fundamental problem is that a whole series of events is misplaced: starting with the murder of his partner (due to a message Carlin had sent into the past) and going through the destruction of the bomber's property as Carlin rescues the girl, and then his subsequent visit to her flat to clean up his injuries (leaving bloodstained dressings), during which the girl rings up the ATF office to check on his identity. These events only occur because of Carlin's efforts to alter the past, and therefore belong in TL2, but they are all observed in TL1, in which the girl died before most of the events which "involve" her. That makes no sense at all.
Yes, I know that the whole premise of the story is impossible anyway, but that isn't the point. I am willing to suspend disbelief and go along with all sorts of impossible plot lines provided that they are internally consistent (I wouldn't otherwise be able to read SFF at all). It's when they lack internal consistency that I lose patience. I am frankly amazed that an entire team of people spent months working on this film and didn't notice, or decided to ignore, these inconsistencies. Either they suffered from collective stupidity, or they assumed that their audience would be too stupid to notice. Not the way to give SF films a good name.