We open with voice-over from Barry Allen, narrating and introducing his speedy abilities over a sequence of him zooming around town stopping generic problems like a girder falling and a mugging. Flashback to two weeks earlier, the ever-late Barry is a CSI police detective in Central City, which neighbors Keystone. Barry is infamous for taking on open cases, aiming to help victims find justice, very similar to the characterization seen in Arrow. Barry is called to a crime scene on the river, where a Jane Doe has washed ashore with no signs as to how she died. Later, Barry meets up with Iris West, his childhood friend and unrequited crush, who has lost her job as a newspaper reporter. She invites Barry to dinner, but he declines.
That evening while examining the dead body, Barry flashes back to his childhood, when his mother pushes a Jay Garrick comic over the textbook hes reading, telling him, sometimes you have to step away from a problem to see it more clearly. She takes him for ice cream, where they see in the distance the legendary scientist Dr. Eobard Thawnes tower and talk about his building a particle accelerator. Back in present day, Barry theorizes the victim couldve been exposed to extremely cold temperatures, but when he approaches Police Chief Chyre requesting a mass spectrometer to confirm it, Chyre tells him to get some sleep and get laid.
But Barry is driven to avenge his victim, and ends up sneaking into S.T.A.R. Labs with a school tour to find the spectrometer. Hes caught by an employee, Valerie, who kicks him out, but feels bad doing so. Barry returns home to find hes forgotten Iris was having a party, and runs over to her apartment to find its an engagement party shes marrying a banker named Nathan Newbury. A ten-year-old Wally West cameos briefly before being sent to bed. Barry starts to leave before Iris catches up to him. Barry congratulates her but leaves full of regret; Iris heads back inside with a brief look of disappointment.
Returning to his place, Barry finds Valerie has sent him the data he needed on the victim, but when a rainstorm blows the window open, a bolt of lightning cracks through it and sends Barry flying across the room. Mid-blast, Barry again flashes back to his childhood, he and Iris playfully flirting as children, his father losing his job, and opening his front door to find his mother murdered and his father blamed for it, before being sent to live with Iris family. In the present, Barry is rushed to a hospital, where he wakes up to find hes seeing things in alternating slow and sped-up time. Thawne pays a visit to the hospital to take Barry into the care of his scientists at S.T.A.R. Labs.
Valerie and a group of other scientists begin to test Barrys abilities, collecting data on the speed Barry travels and the calories he expends. Barry outruns a dragster, clears a canyon, heals two compound fractures in hours, and even stops a nearby tornado by running against its spin. He even finds time to send Chyre the info on the victim, forcing Chyre to publicly announce there is a serial killer on the loose who is freezing his victims to death. We later learn this killer is Leonard Snart, or Cold, as hes dubbed by the press. Cold kidnaps Chyre and kills him on live TV before threatening to destroy a major Central City landmark in five days. Barry resolves to help out with the crisis, but not before Thawne promises to use his developing particle accelerator to help Barry reach an alternate universe where the people hes loved and lost may still be alive.
Iris, now an independent blogger, discusses the pattern to the Cold killings with Barry. We learn that it is the train station Cold is targeting, and when Barry shows up to stop him, Cold sabotages a train full of people to ensure his escape. Barry is forced to pull each individual passenger off the train one by one. But in examining the security cam footage in the tunnel, Iris thinks she sees a mans foot within the tornado pulling people off the train. At dinner with Nathan, Barry mocks her theory, further tensing their relationship.
Iris takes to her blog to talk about the guardian angel that saved the people at the station, whilst Valerie and the gang at S.T.A.R. fashion Barry with a familiar red costume, unbeknownst to Thawne. The team hits the bar afterwards, and when Iris and Nathan see Barry out with Valerie, she becomes jealous. That night, Barry and Valerie have sex. The following morning, Barry and Iris again discuss Cold, determining that he isnt a serial killer, but a hitman working for someone. Barry gets a call and learns Colds next target is Nathan, who is soon frozen and near death. When Flash shows up to rescue Nathan, Cold attacks Iris, but Flash bests him, confirming to Iris hes real. Snart is taken into custody, and Flash leaves Nathan with paramedics and provides a handwritten set of instructions on how to treat him.
Iris goes to Barrys house and says she recognized his handwriting on the note, revealing him to be the guardian angel shes been writing about. They share an intimate moment, but Barry steps back and tells her she should be with Nathan. Barry goes to interrogate Cold, who cryptically announces, hes come for me, before Barry is punched at superspeed by an unknown black-suited speedster. The two fight, and Barry learns this speedster is the one responsible for killing his mother and framing his father. But the villain beats him, leaving Barry alive with the mandate for him to stop running.
Later, Nathan admits to Iris he laundered money for what were later Colds victims, and all the money traces back to S.T.A.R. Labs. Iris is disgusted. Valerie learns the particle accelerator has a secret function to steal away Barrys speed. Hearing this from her, Thawne is revealed to be the dark speedster, and stands up out of his wheelchair and kills her. Learning this, and fearing Thawne will kill Iris if he runs again, Barry wants to stop being Flash. But once all the criminals he put away in Iron Heights prison, including Cold, are released, and bombs have been planted all over the city, Barry becomes Flash once more, defusing the bombs and stopping the criminals. Cold kidnaps Iris, while Thawne explains to Barry that they were mortal enemies in another reality, so he used his accelerator to travel to a new one and slowly destroy Barrys life, then steal his speed. Cold attempts to shoot Iris with a cold needle, but Flash backhands it into Colds chest, and Cold dies feeling warm.
Meanwhile, the particle accelerator begins creating a black hole and sucking everything into it, fulfilling Thawnes plan to destroy this universe and travel to a new one. Flash stops it in the same way he stopped the tornado running counter to the flow, all while pushing Thawne back into the Speed Force. Barry is briefly united with his aged mother and father in the Speed Force before Iris revives him. With the crisis averted, Iris waxes poetic about Flashs heroism while Barry monologues in voice-over while running through the city.
Finally, in a post-credits sequence, Flash skids to a halt after seeing something in his path. Its Green Lantern, who tells him, Lightspeed. Not bad.