Season 4 begins with a powerful armored villain threatening to level Central City if the Flash doesn't appear, so Cisco makes a risky decision to break Barry out of the speed force. But the Barry that comes out isn't the same Barry that went in.
Barry takes on a dangerous meta who can control technology and confronts an obstacle in his personal life: the ramifications of abandoning Iris for six months to balance the Speed Force. Meanwhile, Gypsy prepares for a hot date with Cisco, but she gets annoyed when his work keeps them apart.
Barry and the team are hit by a string of bad luck and realize it is the handiwork of a new meta nicknamed Hazard, who has the ability to give others bad luck while cashing in the benefits for herself. Meanwhile, Harry Wells returns to Earth-1 to give Wally a message from Jesse.
Caitlin receives an ominous threat from her old boss, Amunet, and fears her past time as Killer Frost may be back to haunt her. Meanwhile, Felicity comes to Central City to help the girls celebrate Iris' bachelorette party; and Cisco, Joe and the guys take Barry out for a night on the town.
Barry turns to a surprising ally for help when collectors of Native American artifacts are attacked by a meta who can bring inanimate objects to life. Harry enlists Cisco's help to summon the ultimate think tank to crack the identity of the Thinker.
Part 3 of 4. Barry and Iris's wedding brings the gang together, but things go awry when villains from Earth-X attack the ceremony. All of the superheroes join forces to take on their most formidable villains yet. Earth's mightiest heroes–Green Arrow, Supergirl, The Flash and White Canary–lead their teams into battle to save the world.
Amunet kidnaps Caitlin and puts meta-dampener handcuffs on her to keep her from turning into Killer Frost. Amunet tells Caitlin she needs her to perform a tricky medical task and if Caitlin fails, Amunet will kill her. Meanwhile, the Thinker traps the Flash in a speedster-proof prison. With the clock ticking, the team doesn't have the time or resources to track both Caitlin and Barry, so Iris is forced to choose who to save.
When a familiar villain returns to terrorize Central City, Ralph must rise up to defend the innocent while Barry is detained by his trial. Meanwhile, Barry searches for the strength to keep his optimism alive in the face of his new circumstances.
A meta who can shrink anything he touches battles Team Flash. Cisco and Ralph get caught in his crossfire and are shrunk to miniature versions of themselves. Cecile realizes her pregnancy has caused her to have temporary powers and discovers she can hear other people's thoughts, which unnerves Joe. Meanwhile, Barry meets someone with a mysterious connection to Henry Allen.
After hearing that Amunet has struck a deal to buy all of the metahumans in Iron Heights, Barry must decide if he should expose his powers in order to save his cellmates. After a visit from an old friend, Ralph is reminded of his shady past and wonders if he's really a changed man.
Barry meets a powerful woman whose abilities could help him in his battle with DeVoe. Meanwhile, after learning that DeVoe is targeting everyone who was on the bus, Ralph decides he doesn't want to be a hero anymore, his only concern is staying alive. Harry offers to help Joe
When DeVoe assaults an A.R.G.U.S. facility to complete his Enlightenment Machine, Barry realizes the only way he can stop him is if he allows Cisco and Caitlin to accompany him into the facility. Still shaken by Ralph's death, Barry isn't sure he wants to risk any more of his friends' lives and considers taking on DeVoe solo.