Barry and the team go after the Mist, who possesses toxic gas powers, and they reflect on the night Caitlin's fiancé died. Meanwhile, Henry gets a long-awaited visit from Joe, and Iris and Eddie continue to keep their romance a secret from Joe.

Barry is thrilled when Oliver, Felicity and Diggle come to Central City to investigate a case involving a deadly boomerang.

The crossover continues on Arrow S03E08 The Brave and the Bold.

Barry enjoys the annual tradition of tree trimming and gift exchange with Joe and Iris. He gives Iris a touching gift, but the moment is interrupted by Eddie’s arrival.

While Barry, Harrison, and Caitlin hunt down Ronnie, who has attacked a man, Joe recruits Cisco to help him learn the truth about who murdered Nora Allen.

Martin and Ronnie are separated, only to realize that they must learn how to rejoin before Eiling hunts them down and kills them... and the Flash. Meanwhile, Joe shows Barry the 3D images from the Nora Allen crime scene, and Barry asks Martin for advice on time travel.

When Harrison activates the particle accelerator, the team has no choice but to evacuate the five metahuman prisoners. Unfortunately, with time running out, Barry has no choice but to turn to Captain Cold for help. Meanwhile, Eddie escapes but has some surprising news for Iris.

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.

Picking up months after the Singularity attacked Central City, Barry is still struggling to forgive himself for Eddie’s death. Concerned about putting his friends in danger, Barry has pushed everyone away and has chosen to protect the city on his own. When a meta-human named Atom Smasher attacks the city, Iris tells Barry that he needs to let his friends help him protect the citizens of Central City. Meanwhile, Cisco helps Joe with his Meta Task Force.

Desperate to stop Savitar and save his friends, Barry turns to the Speed Force for answers. H.R. gives Jesse some advice.