After emerging from a coma, Barry discovers that he has superpowers and puts them to the test. However, he soon goes up against another metahuman created by the same accident that gave him his abilities.
Iris is escorted by Barry to a university gathering, honouring Simon Stagg, a famous scientist. When six gunmen interrupt proceedings, Barry changes into the Flash only to pass out. While the scientists try to discover what's wrong with Barry, they realize that they're dealing with a metahuman who can replicate himself. Meanwhile, Iris becomes intrigued by the mysterious red streak fighting crime in Central City.
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. While investigating the case, Barry gets a surprise visit from an old friend - Felicity Smoak, who heard about his new abilities and came to check them out for herself. She joins him at S.T.A.R. Labs to help stop Snart after it's revealed that Snart his gotten 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. Finally, The Flash and Captain Cold have an epic confrontation.
A thief blows up a floor of an office building stealing a government file, and General Wade Eiling arrives in Central City to oversee the investigation. Harrison has a history with Eiling and warns the team not to trust him, and they soon discover that the thief is a metahuman... and Eiling wants her as a weapon. Meanwhile, the Flash meets with Iris to try to convince her to stop blogging about him.
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.
While battling a metahuman who feeds on energy, the Flash loses his powers. The scientists try to restore Barry's powers, just as the same metahuman arrives at S.T.A.R. Labs eager for revenge on Harrison. Meanwhile, Iris and Joe find themselves in a hostage situation when Arrow villain Clock King takes over the police station.
The Arrow and his team come to Central City to identify a boomerang used in a murder. However, they soon end up helping the Flash when he goes up against a metahuman who can drive his victims into fits of rage... . and the speedster is his next target. Meanwhile Iris finds herself caught between her boyfriend and her idol when Eddie tries to create a task force to bring down the Flash.
The man who killed Nora Allen finally confronts the Flash, and soon reveals that he's the hero's superior in every way. Meanwhile, Barry confesses a secret to Iris, Eddie asks Iris to move in with him, and Caitlin discovers that Ronnie is alive... but different.
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 decides that she and Barry need to move on from Ronnie and Iris and find new loves so she takes him for a night out at the local karaoke bar. Caitlin doesn't have any luck making a love connection but Barry meets Linda Park, a sports reporter for the Central City Picture News, and asks her out on a date. When Barry tells Iris he has a date, Iris is surprised by her reaction. Meanwhile, Cisco considers Hartley's dangerous offer and the team searches for Shawna, a meta-human with teleportation powers, who just busted her boyfriend Clay out of Iron Heights. When Henry snoops around in an attempt to help Joe and Barry solve the crime, he ends up in the infirmary after getting roughed up by inmates.
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. They deduce the best way to track Ronnie is to find Dr. Martin Stein who was working on a project called F.I.R.E.S.T.O.R.M. Barry struggles to balance his duties as The Flash and his relationship with Linda Park, and 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.
The Flash learns that Captain Cold and Heat Wave have returned to Central City. This time Snart has brought along his baby sister Lisa 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.
Note: Flash 1x18 takes places before Arrow 3x18.
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.
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.