In Algiers, New Orleans, an explosion of a ferry transporting sailors from the USS Nimitz and their families on a Mardi Gras excursion causes 543 casualties. Lonely ATF agent Doug Carlin (Denzel Washington) is assigned to investigate the terrorist attack.
He is informed by Sheriff Reed (John McConnell) about the corpse of a woman, Claire Kuchever (Paula Patton), that washed up on shore. Her body was found one hour before the explosion, but burnt with the same explosive. The agents believe that her death was meant to look like part of the explosion, but the body arrived too soon. Doug also learns that his partner, Agent Larry Minuti (Matt Craven), was evidently killed in the bombing.
Doug is invited by FBI Agent Pryzwarra (Val Kilmer) to join an experimental surveillance team led by Jack McCready (Bruce Greenwood). The team shows him moving images of the ferry boarding, from any and every angle. He watches the hundreds of people moving around, and after a few minutes starts firing questions at the team about a technology that can provide this type of live surveillance from four days ago (4 days, 6 hours, 3 minutes, 45 seconds). They explain that it is via a system of satellites, and that it takes four days to assemble the information so that it can be viewed just one time. He asks pointedly how satellites are providing audio, and the team looks startled but provides an explanation.
Doug directs them to focus on the mysterious dead woman, Claire Kuchever, and follow her last days since it is likely she was murdered by the bomber. As they observe her, she obviously has the sensation that she is being watched, calling out, "who's there?" and looking around. She even writes about it in her diary, which Doug holds and reads four days later, after it was collected as evidence from her apartment. She receives a call from the terrorist, asking about purchasing her car. However, she will be busy when he wants to meet, so they do not make a deal. As the surveillance continues, Doug watches her getting dressed, and later reading to her niece, he suddenly raises a laser pointer and directs it at the viewing screen. It shows on the wall behind her, and she sees it and reacts to it. Meanwhile, the viewing room system goes down completely, with several people shouting, "who breached the field?!"
Doug angrily confronts them for lying about the system. He knows that the images are not from the dead past because Claire reacted to the laser light. He demands an explanation. The scientists try to tell him about an Einstein-Rosen bridge, or spacefolding technology, but the advanced physics jargon just infuriates Doug. Dr. Denny (Adam Goldberg) tries to make it simpler, showing a piece of paper and then folding it to bring two points together. This is still not what Doug wants to know. He smashes a nearby monitor and demands, "is she alive or is she dead?" The group gives both answers - she is alive in the past and yet dead in the present.
He wants to prevent her murder, and the ferry explosion, but the team explains to him that it cannot be done. Nothing can be sent back - gerbils, all sorts of experimental animals have arrived dead. Attempting to "breach the field" causes failures of the local power grid. A major blackout of most of the central US, that was blamed on Canada, was actually the fault of operator Gunnars (Elden Henson). However, they eventually agree to try to send a small piece of paper back in time to warn Carlin himself, as an anonymous tip, where the terrorist will be waiting to arrange his car deal. The scientists chatter about different time paradox theories. Operator Shanti (Erika Alexander) draws a diagram showing a "branching" theory - a new timeline is created with the new event (the appearance of the paper), and the old timeline ceases to exist. However, Dr. Denny insists it won't change anything. Shanti likens it to throwing a pebble into water, causing ripples ... but Dr. Denny says the flow of time is more like the Mississippi. Any change will simply heal itself, in the long run.
They prepare the message and send it back. Unfortunately, their timing is off. Before the message appears, Doug (in the past) finishes an argument with his partner Larry and walks out of his office. Afterward, Larry spots the paper on Carlin's desk and leaves to follow the tip which places the bomber at the ferry dock. The team cringes as they realize they have sent Larry to his death. They focus now on the dock, hoping to get a good picture of the terrorist so that they can find him. His picture is put into a computer to compare it with mug shots, looking for a match.
Larry meets the terrorist and a brief fight ensues. Unconscious, Larry is dragged into the terrorist's vehicle, and he drives away. As they follow him, the viewing team exclaims that he is leaving their viewing area and will soon be out of reach. Doug fires up the HumVee that contains the mobile viewing rig, giving chase in the present so that the past in that area can be seen. After an accident, the rig is damaged so that Doug cannot see the past through it, but the base team can. They shout instructions directing him. He follows the trail to a burned-out fishing camp on the bayou. There is an ambulance crashed into the remains of the main building, but in the past, the terrorist has taken Larry out back. Pryzwarra gulps that the terrorist is pouring "accelerant" (fuel) on Larry's body. In the present, Carlin can see a burned outline of a body on the pavement. He tells them to brace themselves because they are about witness a murder. Horribly, Larry begins to awaken. The terrorist sees this, and quickly shoots Larry before setting his body on fire.
The team isn't sure what else there is to do. Doug points out that the terrorist still needs a vehicle for his bombing, because there are bulletholes in the one he'd planned to use. That means the terrorist will need Claire's car. Doug continues to examine the site. Inside the building, he finds another burned body outline. In a crack, he finds a black earring which he recognizes as Claire's. He kisses it and pockets it.
The computer search comes up with a match on the terrorist's picture - Carroll Oerstadt (Jim Caviezel). He is connected to the fish camp. He is hunted down, captured and brought in for questioning. Discussing his past with Doug, it seems the military rejected him twice, finding him mentally unstable. Oerstadt babbles about destiny, patriotism and collateral damage.
McCready advises the team that they are shutting down, having caught the perpetrator. There is no point in further pursuing Claire's murder and the explosion. Doug is frustrated - for once he would like to do more than catch the bad guy afterwards - he'd like to stop the bad guy before it happens. He stops at Claire's house and sees the magnets on the refrigerator spell out, "u can save her".
Dr. Denny agrees to help Doug attempt to travel into the past. Doug strips down to his undershirt and boxers and climbs into the small transport area, hugging his knees. Denny reminds Doug that his heart will be stopped by the electromagnetic pulse (EMP) of the system - if he is not revived, he will simply die. He arrives four days in the past, on an empty gurney in a hospital operating room. Seeing his jolting body, doctors rush to treat him. When they cut away his shirt, they read the words "Revive Me" written on his skin. He awakens later in a hospital room, steals some clothes, and grabs a bouquet of flowers to carry for camouflage. Outside the hospital, he steals an ambulance and drives out to the fish camp, where Claire is tied up with a bag over her head. Trying to fight her captor, Oerstadt, she rakes his face with her nails. Furious, Oerstadt douses her with gasoline. Doug crashes the ambulance directly into the building where they are. Oerstadt shoots Doug in the shoulder but then flees.
Doug takes Claire home to clean up. He looks around her apartment, seeing all of the details that he has seen through the viewer and also from the investigation after her death. He shifts the refrigerator magnets to spell "u can save her". While he is cleaning up, she comes out wearing the red and white dress in which she died. He tells her to change, stating that is what "the victim" wore. He reaches into a wardrobe for clothing - and she pulls a gun on him, stating that he cannot be what he claims, pointing out his familiarity with her house and the fact that he has asked her no questions. She calls the ATF and they confirm his existence and appearance, but she demands to know more. He says, "What if you had to tell someone the most important thing in the world, but you knew they'd never believe you?" She says, "I'd try." After explaining, she helps bandage his shoulder.
They go to the ferry, where he will defuse the bomb and Claire is supposed to tell the guards what is happening. Unfortunately, when Oerstadt leaves the ferry and sees Claire's truck, he knows he has a problem, so he returns to the ferry. Claire sees this and jumps aboard also, just as the ship leaves the dock. Doug returns to the bomb-laden vehicle to find Claire in it, handcuffed to the steering wheel, with her mouth taped shut. Doug stuns Oerstadt into a standoff by throwing his own words (from the interrogation) at him: destiny, patriotism and collateral damage. Rather than surrendering, Doug jumps into the car with Claire and gets her to ram Oerstadt where he stands in front of another car.
However, the car is still wired to explode, and there is no time to defuse it nor explain all of this to the guards. Doug directs her to drive the vehicle off the ferry and into the water. As they submerge, he kicks out the windshield and jerks the steering wheel out of the column so that she can swim free to the surface. Unfortunately, the ferry continues overhead, rolling the vehicle over and crushing the frame so that Doug cannot escape. Claire makes it to the surface, gasping, and is pulled out of the water. The car is completely filled with water, and Doug hammers futilely on the side window, holding his breath. The ferry safely passes. The vehicle explodes underwater, sending a fireball of debris up in the air. Now ashore and wrapped in a blanket, Claire cries over Doug's death.
The officials ask Claire if she would talk to an agent from the ATF. She turns ... and sees Doug (from the present) walking up to her. She stammers in shock, and he asks if they have met. As they get into his car to leave, she repeats his words from earlier in the film, "What if you had to tell someone the most important thing in the world, but you knew they'd never believe you?" And he unwittingly echoes her earlier response, "I'd try."