In the series premiere, Central City forensic scientist Barry Allen (Grant Gustin) is accidentally given the power of super speed in a freak particle accelerator accident.

Barry escorts Iris to a university gathering honoring scientist Simon Stagg. When six gunmen storm the event, Barry changes into The Flash and tries to stop them. While he does save a man's life, he passes out before he can capture the robbers, which frustrates him. As Dr. Wells, Caitlin and Cisco scramble to find out what's wrong with Barry, Joe comes down hard on Barry for taking the law into his own hands and risking his life. Barry realizes that it wasn't six gunmen but a metahuman named Danton Black, who can make multiples of himself. Meanwhile, Iris becomes even more intrigued by the "red streak."

As Barry and the team at S.T.A.R. Labs work to capture Kyle Nimbus, a.k.a. The Mist, a dangerous new meta-human with toxic gas powers, they revisit the painful night the particle accelerator exploded and killed Caitlin’s fiancé, Ronnie. Meanwhile, Joe decides to finally visit Henry in jail after all these years, but things take a dangerous turn when Kyle shows up looking to punish Joe for arresting him years ago. Meanwhile, Iris and Eddie continue to hide their relationship from Joe.

The Flash stops a robbery but the culprits get away after shooting a guard, and The Flash chooses to save the man instead of following the criminals. Joe shows Barry a book of suspects and Barry identifies Leonard Snart as the leader of the group. Snart revises his plan to steal the Kahndaq Dynasty Diamond and gets a boost when he gets his hands on a stolen "cold gun," which could kill The Flash. Dr. Wells is furious when he finds out that Cisco built the cold gun without telling anyone and now it's missing. Meanwhile, Iris is getting the silent treatment from Joe because of her relationship with Eddie.

After a bomb goes off downtown, the army, led by General Eiling, rolls in and takes over the case, much to Joe's surprise. Suspicious, Joe tells Barry that he and his friends at S.T.A.R. Labs should look into the army's involvement. Wells informs the team that Eiling was experimenting on his men to turn them into super soldiers. Cisco confirms one of Eiling's soldiers, Bette Sans Souci, was at the bomb site. The Flash tracks her down and realizes that she's not setting off the bombs, she is a meta-human who can blow things up just by touching them. Meanwhile, when Joe learns Iris is writing about "the streak," he tells Barry to make her stop. Barry realizes Iris won't listen to him so he decides to have The Flash pay her a visit.

The Flash faces a new meta-human named Tony, who can turn himself into girded steel at will. While Dr. Wells and Caitlin are concerned about Barry’s safety, Cisco comes up with a plan to take Tony down. Meanwhile, Iris’s blog on The Flash gets her into trouble, and Eddie witnesses Tony’s abilities firsthand and begins to ask questions that Joe doesn’t want answered. Joe asks Dr. Wells to help him solve Nora Allen’s murder.