Until recently, 26 year-old Barry Allen (series star GRANT GUSTIN) lived a normal life as a C.S.I. in the Central City Police department. He was secretly in love with his best friend, Iris West (series star CANDICE PATTON), daughter of Barry's surrogate father, Detective Joe West (series star JESSE L. MARTIN). Joe adopted Barry fifteen years ago after Barry's mother was murdered and Barry's father, Henry Allen (recurring guest star JOHN WESLEY SHIPP), received a life sentence for the crime -- though Barry always maintained that a mysterious "Man in Yellow" was responsible. Then the S.T.A.R. Labs Particle Accelerator exploded, creating a dark matter lightning storm that affected Barry, bestowing him with superspeed and making him the fastest man alive. But Barry wasn't the only person who was given extraordinary abilities that night. The dark matter also created meta-humans, many of whom have wreaked havoc with their powers on the city. With the help of his scientist friends at S.T.A.R Labs, Caitlin Snow (series star DANIELLE PANABAKER), Cisco Ramon (series star CARLOS VALDES), and Dr. Harrison Wells (series star TOM CAVANAGH), Barry began a journey as The Flash to protect the people of Central City from these powerful new threats. With this team, Barry was finally able to defeat the Man in Yellow, aka The Reverse Flash, but in their epic battle, a Singularity was ripped in space and time that threatened to destroy them all.
After successfully closing the Singularity from destroying Central City, Barry thinks he's seen the worst... but the arrival of Jay Garrick (recurring guest star TEDDY SEARS) reveals that the Singularity actually opened portals to an alternate Earth, known as Earth-2, which is being terrorized by a formidable evil speedster named Zoom. As Zoom sends an army of Earth-2 meta-humans to Earth-1 to defeat Barry, he receives unexpected assistance from the Earth-2 doppelganger of his former mentor, Harrison Wells. Meanwhile, Cisco discovers that he, too, was affected by the dark matter of the Particle Accelerator and must come to terms with his newfound powers. As Barry struggles to juggle life as a hero, he realises just how hard it is to find happiness when the lives of everyone you love are at stake. He finds some solace in the companionship of his father, Henry Allen, who was freed from a wrongful life sentence... but when Zoom kills Henry before Barry's eyes, Barry's newfound stability is shattered.
Blinded by anger, Barry unwittingly plays into Zoom's game and uncovers the evil speedster's true goal: to destroy all Earths in the multiverse. In the race of his life, Barry ultimately gets the upper hand against Zoom and defeats his nemesis. But unable to celebrate victory, Barry makes a world-shaking decision and speeds back in time to the night his mother died to stop The Reverse Flash from killing her, irrevocably changing his past and determining his future anew.