Mark Mardon, having survived his apparent death during the black-matter explosion, returns seeking revenge on Joe for the death of his brother Clyde... and has the same weather-manipulation powers as his deceased sibling.

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.

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.

After recent events, Barry decides it’s time to confront Zoom and comes up with a dangerous plan. Caitlin sides with Jay and deems it too dangerous for Barry to engage with the speed demon. However, Iris shows her support and brings forth a surprising ally. We see what Earth-2 Harrison Wells’ life was like before coming through the breech.

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.

Barry, Wells and Cisco journey to Earth-2 to rescue Wells’ daughter, Jesse from Zoom. Barry is stunned when he runs into Earth-2 Iris and Joe, but nothing prepares him for meeting Killer Frost and Deathstorm. Meanwhile, back on Earth-1, Jay has to take over the Flash’s responsibilities when a meta-human nicknamed Geomancer attacks Central City.

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.

When a villain calling himself the Trickster runs rampant in Central City, Joe and Barry go to Iron Heights to talk with the original Trickster.

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.

On Earth-2, the team races to find Zoom's lair and asks for help from the most unexpected source. Meanwhile, back on Earth-1, Caitlin rushes to perfect Velocity-9 so that Jay can stop the Geomancer.

After Zoom reveals his true plan, Barry vows to do whatever it takes to stop him.

Team Flash throws a surprise birthday party for Barry, but things go terribly awry when Ramsey Rosso crashes the festivities.

Joe and Cisco go to Starling City to ask Quentin Lance for his help investigating the site of Harrison's car accident 15 years ago and determining what really happened. Meanwhile, Eddie is framed for a shooting by a shapeshifing metahuman who can take on the appearance of anyone he touches.

Vandal Savage arrives in Central City and sets his sights on Kendra Saunders. After Vandal attacks Kendra and Cisco, they turn to Barry for help. Realizing how dangerous Vandal is, Barry takes Kendra to Star City and asks Oliver and team to hide her until he can figure out how to stop Vandal. However, things quickly go from bad to worse when a man with wings AKA Hawkman shows up and flies off with Kendra. Meanwhile, Harrison develops a serum to make Barry run faster and asks Jay to test it out. The crossover continues on Arrow S04E08 Legends of Yesterday (II).

After the shocking revelation that Zoom is Jay Garrick, Barry is more determined than ever to get back to Earth-2 to stop Zoom forever. Desperate to find a way to increase his speed, Barry decides to travel back in time and masquerade as his earlier self in order to get his arch-nemesis, Dr. Harrison Wells/Eobard Thawne, to teach him how to run faster. However, things don’t go as planned and Barry is stunned to face familiar foe Hartley Rathaway, as well as old friend Eddie Thawne. Iris makes peace with her past in order to embrace her future.

After Zoom unleashes an army of Earth-2 meta-humans on Central City, Barry is shaken when he sees their leader is the Black Canary’s Earth-2 doppelganger, the Black Siren. Meanwhile, Wally takes to the streets to help The Flash stop the meta-humans, which worries Joe. Iris and Henry are concerned about Barry taking on Zoom.

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.

Zoom arrives back on Earth-1 intent on taking over Central City. Barry and Wells come up with a plan to stop Zoom once and for all but it’s extremely dangerous. Unsure if he should take the risk, Barry reaches out to both fathers for advice. Henry is adamantly opposed to Barry risking his life again but Joe thinks he can handle it which puts the two men at odds with each other. Meanwhile, Cisco is shocked when he vibes the Earth-2 villain Rupture, who happens to be his brother Dante’s doppelganger. Rupture came to this Earth seeking justice for Reverb’s death. Iris decides she’s finally ready to open up to Barry about her feelings for him.

As Iris receives word of a career milestone, Barry is suddenly nowhere to be found. In trying to get back home, Barry runs across many familiar faces. Khione continues to figure out her powers, and Chester works on a suit for Allegra.

When Cisco gets a vibe of Eobard Thawne, Barry and the team don’t believe it. But, after an attack at Mercury Labs, Christina McGee confirms that the Reverse Flash is back. Meanwhile, Iris and Francine share a nice moment that brings Iris closer to her brother, Wally.

With Barry gone, the team must figure out a way to handle the return of an old enemy - Girder. Realizing Girder is retracing his steps from his last attack, Iris volunteers to act as bait to trap him in S.T.A.R. Labs. Meanwhile, Barry fights to return to his old life.

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.

Jay Garrick, a mysterious man from Earth-2, appears at S.T.A.R. Labs with a dire warning about an evil speedster named Zoom, who is set on destroying The Flash. Barry and the team must decide if they can trust this stranger even as they face yet another powerful meta-human. Meanwhile, Joe must deal with a determined officer named Patty Spivot who wants to join his meta-human task force.

Barry learns a new breacher, Dr. Light, has come through the portal and sets off to capture her. Jay tells Barry that Dr. Light was not a threat on Earth-2 and that Barry can reason with her. However, during a fight with The Flash, she blinds him and drops some shocking news about Zoom. Meanwhile, Barry and Patty go out on a date.

The Flash battles Captain Cold after the villain is involved in a robbery and plots to steal the Kahndaq Dynasty Diamond.