Lou Bloom (Jake Gyllenhaal) wasn't always a stringer. His resume includes petty theft, most recently stealing from a construction site in downtown Los Angeles. Challenged by a security guard he overpowers him and takes off, but not before he steals the guard's wristwatch. Lou sells the construction materials at a local scrapyard. Lou drives home after the transaction and finds his attention diverted by a car wreck on the other side of the street. He pulls over to get a better look at what is happening and sees police officers rescuing a woman from the mangled car. He also sees a group of photojournalists - known as stringers - filming the action. One of them comes over to Lou and tells him that they make a lot of money selling the footage to local television news stations.
Lou is immediately captivated. He steals a bicycle and pawns it, exchanging it for a video camcorder and a police radio scanner. Like every good ambulance chaser he is now prepared for the job. After a couple of false starts he finally manages to film a crime as it is happening, recording usable footage of a carjacking in which the victim is killed. He contacts KWLA 6 and sells them the footage. They are impressed; morning news director Nina Romina (Renee Russo) calls Lou and tells him that the station will always buy footage of violent incidents, especially those that take place in affluent, "safe" areas.
Lou being Lou, he cannot be a stringer in a legal and ethical way. He thinks nothing of tampering with crime scenes if he thinks it will make his what more impactful and shocking and he even moves the position of a body to capture the injuries more effectively. He becomes well-known in stringer circles, and sells a lot of footage to Nina, which means that he makes more money, and enables him to purchase a better camera. He also buys a new car, which is faster than his old car, and enables him to make it to crime scenes more quickly than anyone else.
Lou is attracted to Nina; it's not particularly mutual, but he uses his status as a stringer to put pressure on her to go out on a date with him; he threatens to take his footage to another television station if she says no, and he demands that she sleep with him too. To Nina, this is an offer she cannot refuse, because she wants to stay at the top of the news game and continue to beat out rival stations for audience share. He also begins to commit additional crimes in order to report crimes; he is beaten to the scene of a crime by fellow stringer Joe Loder, which makes Renee very angry She had sex with him, and kept her side of the arrangement. Why is he not keeping up his side of what they agreed? Angry, Lou tampers with Lou's vehicle so that Joe cannot beat him to the next crime scene, but the sabotage causes the van to crash, causing Joe significant injuries. Lou records the crash and sells the footage.
Later that night, Lou and his new assistant, Rick, get to a crime scene before the police do. It is a home invasion in a very affluent neighborhood, and Lou not only gets footage of the gunmen fleeing the house, but then goes inside the home to film the homeowners. This is where the news team begin to divide; some feel that this highly raw, unsanitized scene is crossing the line, but Nina doesn't have a line to cross; she has a story to break and a handy lack of ethical backbone that enables her to go ahead and break it. Lou wants more money, and a credit on the footage. This backfires a little; he comes to the attention of the police, who question him about his involvement in the home invasion. Lou edits his footage and gives Nina a shortened version without the footage of the gunmen running from the scene, but later he and Rick track down the men, hoping to follow them and then call the police so that they can record the arrest on camera.
They follow them to a restaurant; Lou calls the police, whose arrival begins a shoot out. One of the gunmen is killed but the other escapes. The police chase the escapee, and Lou chases the police, recording all the while they are doing so. The gunman's SUV swerves and crashes, and Lou believes him to be dead, urging Rick to film his body. It turns out that he is not as dead as Lou first thought, firing at him as he films but shooting Rick instead. The police then shoot the gunman again, killing him. It is too late for Rick though; he lays dying, and Lou films. Nina is sold on Lou. She is in awe of the footage, but when her team learn that the random home invasion was a drug deal gone bad, she withholds the fact from both the story and the police, because it has more impact if it is a random attack on innocent, well to do people.
The cops also want Nina's footage as evidence but she stands by her rights and the freedom of the press, refusing to hand it over. Lou is also questioned, making up a tale about the men in the SUV following him and Rick, rather than the other way around. Detective Frontieri knows this is a lie, but he has no proof, and cannot arrest him. Lou ends up on top; he hires a group of interns, buys them all vans, and expands his salacious empire.
