CSI investigator Barry Allen awakens from a coma, nine months after he was hit by lightning, and discovers he has superhuman speed.
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.
The Flash goes up against Farooq aka Blackout, a metahuman who can harness electricity. During their battle, Farooq zaps The Flash and siphons all his electricity, leaving The Flash without his speed. Dr. Wells, Caitlin and Cisco work to reverse the effects on Barry, but their efforts are disrupted when Farooq comes banging on S.T.A.R. Labs doors in search of Wells, who he blames for his accident. Without Barry's powers to protect them, the group must figure out a way to save themselves from the metahuman. Meanwhile, Tockman manages a coup inside the Central City police department and takes several people hostage, including Joe and Iris. While Iris is sure The Flash will save them, Eddie has a trick up his sleeve.
Barry is thrilled when Oliver, Felicity and Diggle come to Central City to investigate a case involving a deadly boomerang.
Barry finds himself face-to-face with his nemesis, the man in the yellow suit, a.k.a. Reverse-Flash, who killed his mother. Barry is frustrated when the Reverse-Flash escapes, but Dr. Wells and Cisco come up with a plan to trap him. All they need is bait, so they turn to Dr. Tina McGee from Mercury Labs to help. Meanwhile, it’s Christmas at the West household and Iris is in full holiday cheer. Eddie gives her a surprising gift but also leaves her with a question that could change her life. Joe joins the search for the man in yellow and ends up in the crossfire. Caitlin finds Ronnie.
Leonard Snart AKA Captain Cold returns to Central City with a new hotheaded partner in tow – Mick Rory AKA Heat Wave. The duo plan to steal a multi-million dollar painting, but Cold has another agenda while in town – to set a trap for The Flash. Snart and Mick kidnap someone close to The Flash and threaten to kill them unless The Flash shows up for a battle of fire and ice. Barry tells Joe he isn’t sure he should take on Snart again after the casualties that happened the last time they fought. Meanwhile, Barry asks Dr. Wells, Caitlin and Cisco to help him double his training efforts so he’s ready for the Reverse Flash when he returns to Central City. Iris deals with the aftermath of Barry’s confession, and Cisco makes the CCPD a new shield.
Dr. Wells’ former protégée, Hartley Rathaway, returns to seek revenge on his mentor after being affected by the particle accelerator explosion. Now able to manipulate sound waves, the brilliant Rathaway is a dangerous threat to both Wells and The Flash. Meanwhile, Iris is thrilled when she’s hired by the Central City Picture News as their newest cub reporter. Unfortunately, her editor pairs her with a veteran reporter, Mason Bridge, who wants nothing to do with her. Cisco recalls his first day on the job at S.T.A.R. Labs when he met Hartley and Caitlin. Joe asks Eddie for a favor but asks him to keep it a secret from Barry.
Caitlin takes Barry out to a local karaoke bar to help them both find new loves. Caitlin strikes out but Barry meets a sports reporter and asks her out for a date. Cisco considers Hartley's dangerous offer. The team searches for Shawna, a meta-human with teleportation powers.
After Ronnie attacks a physicist, Barry and the team realize they need to go after Caitlin's fiancé who is now a dangerous meta-human. Barry struggles to balance his duties as The Flash and his relationship with Linda Park. Joe enlists Cisco's help to reinvestigate the murder of Nora Allen. General Eiling returns to Central City.
After the nuclear explosion separates Ronnie and Dr. Stein, Barry and the team believe both men are safe. Caitlin is thrilled to have her fiancé back and prepares to resume their life together while Dr. Stein returns to his wife. When General Eiling targets Firestorm, Ronnie and Dr. Stein must decide if they are safer together or apart. Mason Bridge tells Iris that there is something secretive going on at S.T.A.R. Labs. Dr. Stein gives Barry some important information about time travel.
Mark Mardon aka The Weather Wizard appears in Central City intent on avenging his brother Clyde's death, and sets his sights on Joe, who shot Clyde. When Mardon attacks Joe and Barry, Barry is able to super-speed them to safety but Mardon gets away. Barry warns Joe not to go after a meta-human alone but Joe doesn't listen and ends up in grave danger. Meanwhile, Cisco looks into the night the team captured the Reverse Flash and realizes something doesn't add up and begins to wonder whether Joe was right about Dr. Wells. Barry and Linda end up on an awkward double date with Eddie and Iris.
After going to the past, Barry talks to Harrison Wells about him time traveling and the consequences of changing the timeline. Meanwhile, the Flash learns that Captain Cold and Heat Wave have returned to Central City. This time Snart has brought along his baby sister Lisa aka Golden Glider to help wreak havoc on the city.
A copycat killer who goes by the name “The Trickster” starts setting off bombs in Central City. In order to stop the villain, Barry and Joe meet with the original Trickster, a criminal mastermind named James Jesse, who has been imprisoned for 20 years. Things quickly go from bad to worse when the Tricksters unite and take Henry prisoner. Meanwhile, Iris asks Eddie for help with a case and flashbacks show how Harrison Wells came up with the idea for the particle accelerator.
Barry is surprised when Felicity Smoak arrives from Starling City along with her boyfriend, Ray Palmer, who flies in as the Atom. They’ve come to visit S.T.A.R. Labs as Ray needs help with his suit. Their timing turns out to be fortuitous because it’s all hands on deck after a meta human named Brie Larvin unleashes hundreds of robotic bees to attack and kill her former co-workers, including Dr. Tina McGee. A group dinner with Barry, Felicity, Ray, Iris and Eddie turns into a disaster. Note: Flash 1x18 takes places before Arrow 3x18.
Joe and Cisco head to Starling City to continue their investigation of Dr. Wells. While in town, the duo enlists the help of Captain Lance, and Cisco meets the Black Canary, who asks him for a favor. Meanwhile, back in Central City, Barry races to catch a meta-human named Hannibal Bates who can transform himself into every person he touches – which includes Eddie, Iris, Caitlin and even The Flash.
While in Wells' secret room, Barry learns about his battle with the Reverse-Flash in the future, as well as key moments in his life that include marrying Iris, a promotion at the CCPD, and that he created the artificial consciousness known as Gideon in the future. Barry, Caitlin, Cisco and Joe set a trap for Wells. Cisco uses himself as prey which puts him in grave danger. Meanwhile, Eddie makes a decision regarding Iris, which leaves Joe a bit unsettled.
Barry must deal with the Reverse Flash's latest threat to a member of the group. To make matter worse, Dr. Wells unleashes Grodd on the city in order to distract Barry and the team. Joe, Barry and Cisco head down into the sewers to catch Grodd but the gorilla quickly gets the upper hand after he kidnaps Joe. Iris and Barry have a heart to heart talk.
As Wells once again gets the upper hand on the S.T.A.R. Labs team, Barry realizes he needs to make a big move and reaches out to an old foe, Captain Cold, for help. Joe and Caitlin warn Barry that Cold can't be trusted. True to form, Cold has his own agenda that involves the meta-humans trapped in the containment cells. As things seem to be going from bad to worse, The Flash gets reinforcements - Arrow and Firestorm.
Wells presents Barry with a life-changing choice. Dr. Martin Stein and Ronnie Raymond return to help the S.T.A.R. Labs team with this final fight.