On the morning of Mardi Gras (Fat Tuesday), an American Terrorist sets off a bomb in an SUV aboard a ferry, killing more than five hundred people aboard, as the fireball sets off another explosion in the boiler, and eventually the remains sink to the bottom of the river.
ATF Agent Doug Carlin (Denzel Washington) is assigned the grisly task of figuring out exactly what happened, and as such, he's one of the first on the scene collecting evidence. His partner, who had just taken off for vacation, is nowhere to be found, not even answering his cell phone.
Before long, Carlin is assigned to a special task force researching the event, and finds himself drawn into a special new technology that allows the investigators to look back into the past four-and-a-half days (technically, four days and six hours, but no one seems to notice this, so I'll pretend not to do so either) to see what's happening. The only catch is that this time window is open only for that exact period, and once that time window is gone, it can't be recovered.