As Barry and Iris prepare for their new life, luck changes for those in Central City. Cecile takes on a case with Allegra's help, involving a string of unfortunate – and highly unexpected – events. Meanwhile, Chester and the team work with Khione to figure out her abilities.

Iris is visited by Nia Nal, as she needs Iris' help. When Iris and Nia fall into a fever dream and explore different possibilities for their lives, Barry, Chester, Allegra, and Cecile desperately try to help them. Meanwhile, Mark entertains Khione, but they have different ideas of what is fun, leaving Khione to feel that Mark is trying to make her something she is not.

When Ultraviolet returns to Central City, Allegra is determined to find her cousin and change her heart. Meanwhile, Joe finds evidence that Kristen Kramer may not be the good cop she appears to be.

Barry considers a risky plan to save Iris from the Mirrorverse. Ralph attempts to stop Sue from making a grave mistake that could destroy her life.

Having received an ominous threat from her old boss, Amunet, Caitlin fears that her past time as Killer Frost may be back to haunt her. Felicity comes to Central City to help the girls celebrate Iris's bachelorette party, while Cisco, Joe and the guys take Barry out for a night on the town.

The Flash, the fastest man alive, is tasked with his greatest challenge yet, to save the timeline and save existence. Friends old and new gather for an epic battle to save Central City, one last time.

As Eva becomes more powerful, Barry and team must find a way to stop her. They are shocked when an old friend - Sue Dearbon risks her life to help.

Team Flash confront a new bus meta, Matthew aka Melting Pot. He has the ability to swap people’s DNA, so during a battle with Team Flash, he transfers Barry’s super speed to Iris. Now, with a new threat unleashed on Central City, Barry must act as the team leader while Iris takes on the mantle of superhero speedster in order to defeat their new foe.

A seemingly simple mold inspection at S.T.A.R. Labs leads to unexpected time anomalies. Barry and Iris try to make sense of the confusion - and figure out how to get back to their reality. Chester admits a secret to Allegra, which makes things awkward between the two, and Allegra relies on Cecile to act as a buffer. Meanwhile, Khione meets with an old friend of Team Flash.

Barry and Iris plan a romantic dinner for Valentine's Day but their evening is interrupted by an old foe - Amunet; Frost gets into the holiday spirit and attempts to help Allegra reconnect with an old love.

Barry feels guilty for what happened to Caitlin, and Mark comes up with a plan that makes everyone skeptical. Meanwhile, Red Death looms in Central City and commands that failure is not an option. Joe asks Cecile to make a choice that leads Cecile to think about the greater good. Allegra and Chester take a step in a new direction. Lastly, old friends pay an unexpected visit to S.T.A.R. Labs.

As the Red Death wreaks havoc on Central City, Barry and Iris come up with a plan and turn to Cecile for help. Barry is faced with what he thinks was his greatest mistake and the Red Death's greatest success. Khione wants to help Team Flash, but Barry can't risk losing her. Joe gives Barry a much-needed pep talk.

After Black Hole hires Rag Doll to kill Joe, Singh suggests he go into Witness Protection but Joe refuses to stop investigating Carver. While investigating Carver with Cisco, Ralph runs into Sue. Iris becomes suspicious of Eva.

After a devastating betrayal, Barry turns to Timeless Wells for help. Meanwhile, Iris leads Team Citizen down a dangerous road in search of answers, and Cisco confides his biggest fear to Kamilla.

Barry and Iris come together to try and stop a dangerous force from destroying Central City.

Iris is pushed to the limit as she seeks to uncover the mysterious truth about Psych; Barry initiates a new training system, that could potentially backfire, with devastating consequences; Joe and Cisco make life-altering decisions.

The Flash is forced to make an impossible choice. With the power out in the city, Team Flash calls upon the Rogue Squad to help as Iris is visited by an old friend. Joe and Cecile work on finding a balance.

Barry and Iris start to put together the pieces of what's happening in Central City and believe they know what the Rogues are after. Team Flash works together and recruits some unlikely allies to help, and the plan does not go as Team Flash expects. Meanwhile, Allegra does her best to avoid having a conversation with Chester.

Barry and team are surprised when Mon-El and Hank Henshaw arrive on their Earth carrying a comatose Supergirl who was whammied by the Music Meister. Unable to wake her up, they turn to Team Flash to save her. However, the Music Meister surprises The Flash and puts him in a similar coma, one that Team Flash can't cure. Kara and Barry wake up without their powers in an alternate reality where life is like a musical and the only way to escape is by following the script, complete with singing and dancing, to the end. The crossover starts on Supergirl S02E16 Star-Crossed (I).

Cisco's faith in his ability to fill Barry's shoes as Team Leader is upended when he learns of a shocking murder. Meanwhile, Killer Frost trusts no one except herself to hunt down the dangerous Ramsey Rosso.

When Godspeed returns, Barry turns to Hartley Rathaway for help. However, things get tense quickly after Barry realizes one of the changes from Crisis is that The Flash and Pied Piper are now enemies. Meanwhile, Iris tries to escape the Mirrorverse.

Cecile is forced to confront her past in order to break free from a psychic prison. Meanwhile, Chester takes over for Cisco but makes a mistake that puts Barry in serious danger. Joe investigates Kristen Kramer's old military ties and discovers an unsettling truth.

Team Flash uses the distraction of a bank robbing Meta to detract from the grief of losing someone they love.

The Flash goes off the grid to look for answers about a new Meta in town. Meanwhile, Allegra pulls out all of the stops at CC Citizen Media, to protect a source and a friend.

The Flash must deal with Abra Kadabra's sudden return to Central City. The villain is back with a vengeance and a score to settle. Meanwhile, Allegra deals with a tricky situation and Caitlin suspects something is off with Frost. Iris is forced to look at a dark moment in her past.