Barry feels guilty for what happened to Caitlin, and Mark comes up with a plan that makes everyone sceptical. Meanwhile, Red Death looms in Central City and commands that failure is not an option. Lastly, old friends pay an unexpected visit to S.T.A.R. Labs.

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, but the plan does not go as Team Flash expects.

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.

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, but Barry can't risk losing her.

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 very 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.

A seemingly simple mould 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. Meanwhile, Khione meets with an old friend of Team Flash.

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

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.

Iris is alarmed by Barry's disappearance and Cecile assures her everything will be ok, but does she know that for certain? Team Flash is affected by a mysterious substance and Khione develops a better understanding of what she can and cannot control.

Team Flash works together to figure out how to protect Barry, all while being very careful who they trust. Cecile is sceptical of the plan after an unsuccessful attempt. Khione's confidence in Chester enables him to convince Cecile to try one more time.

The Flash 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.