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.

The Flash gets blasted with a shockwave forcing his body to age prematurely.

With a new speedster in town, The Flash gets much more than he bargained for, meanwhile Cecile's powers experience a growth spurt allowing her to aid Team Flash on an entirely different level.

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.

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 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.

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.