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.

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.

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.

While Barry tries to find Eddie, a mysterious mind-controlled thief launches attacks on Central City’s gold reserves. The team soon discovers that a telepathic gorilla is behind the robberies… and is following Harrison’s commands. Meanwhile, Iris confronts Barry, while Harrison prepares to return home.

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.

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.

Felicity and Ray Palmer pay a visit to S.T.A.R. Labs for help getting Ray’s ATOM suit to work, and ask Cisco for help. Meanwhile, a young tech expert raises havoc with her insect drones, and Barry asks Ray for help dealing with her.

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.

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.

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.

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.

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.

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. Meanwhile, Cisco considers Hartley’s dangerous offer; and the team search for Shawna, a meta-human with teleportation powers who just busted her boyfriend out of Iron Heights.

Dr. Wells’ former protégé, Hartley Rathaway, returns to seek revenge on his mentor. Meanwhile, Iris is hired as a cub reporter; Cisco recalls his first day on the job at S.T.A.R. Labs; and Joe asks Eddie for a favor but asks him to keep it a secret from Barry.

While Barry strives to become fast enough to defeat Reverse-Flash, Captain Cold returns to Central City with a partner and takes Caitlin hostage to force a confrontation with the Flash.

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

The Flash goes up against Farooq aka Blackout, a metahuman who can harness electricity.

The Flash faces a new meta-human named Tony, who can turn himself into girded steel at will.

After a bomb explodes downtown, the army rolls in and takes over. When Joe learns Iris is writing about ‘The Streak’ and tries unsuccessfully to have the story stopped, Barry decides to have The Flash pay her a visit.

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

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 changes into the Flash when six gunmen storm a university event honoring a scientist, but his heroics don’t match up to his expectations. Meanwhile, Iris becomes even more intrigued by the “red streak.”

Barry discovers his powers and puts them to the test, only when he finds its no longer a test but the real thing when he encounters a certain someone.

Lex Luthor reaches out from prison to enlist Lena into one of his plans and will stop at nothing to get her on board, even if that means leveraging the life of one of her friends.

With his P.I. shingle newly hung outside his office, J’onn welcomes new clients. Trying to take her mind off what happened at the DEO, Kara decides to team up with J’onn on his latest assignment, which unfortunately ends up tying directly into Alex’s current investigation that involves the villain Menagerie. Meanwhile, Lena shares some news with James, but his reaction isn’t what she expected. Nia invites Brainy to her Valentine’s Day party.

Kara joins Nia on a trip to Nia’s hometown to visit her family during the town’s annual Harvest Festival. While home, Nia’s mother encourages her daughter to embrace her destiny. Meanwhile, Alex deals with a street drug that is turning people violent and giving them temporary superpowers.

Colonel Haley is upset after Supergirl responds to a distress call from a Navy ship that turned out to be a deadly alien attack. Colonel Haley reclaims the site as DEO territory and demands that Supergirl step aside. After Supergirl refuses to be told what to do, Haley refocuses her energy on finding out Supergirl’s true identity and begins to interrogate all DEO agents, including Alex. Meanwhile, Brainiac-5 tries to persuade Nia to embrace her superpowers but she misinterprets his motives.

Supergirl, The Flash, Green Arrow and Superman engage in the battle of their lives.

The crossover starts on The Flash S05E09 Elseworlds (I) and Arrow S07E09 Elseworlds (II).

Nia has a powerful dream about Agent Liberty but refuses to look at it as a prophetic dream and pushes it aside. After noticing something is bothering Nia, Kara enlists Brainy’s help, and the two try to persuade Nia to embrace her destiny. Meanwhile, Manchester Black pays Ben Lockwood a menacing visit.