When a powerful alien threat arrives on Earth under mysterious circumstances, Barry, Iris and the rest of Team Flash are pushed to their limits in a desperate battle to save the world. But with time running out and the fate of humanity at stake, Flash and his companions will also need to enlist the help of some old friends if the forces of good are to prevail.

Barry trusts his instincts during a murder investigation, believing the suspect, despite the overwhelming evidence against him, meanwhile Iris gives Allegra an opportunity to be a mentor.

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

The Flash gets an assist from XS. Meanwhile, CCPD enlists Chester for help when a mysterious device is found at a crime scene.

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.

Barry creates a map book to guide him and Iris throughout their future in order to keep her safe, but the results are not what he expected, and instead, they relive the same day over and over again. Joe has a heart-to-heart with Cecile. A new big bad is introduced to Team Flash and friends and foes, old and new, begin to descend upon Central City.

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.

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.

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.