Don't Run

"Don't Run" is the ninth episode and midseason finale of the fourth season of The Flash, and the seventy-eighth episode overall. It aired on December 5, 2017.

Synopsis
Amunet kidnaps Caitlin and puts meta-dampener handcuffs on her to keep her from turning into Killer Frost. Amunet tells Caitlin she needs her to perform a tricky medical task and if Caitlin fails, Amunet will kill her. Meanwhile, The Thinker traps The Flash in a speedster-proof prison. With the clock ticking, The Team doesn’t have the time or resources to track both Caitlin and Barry so Iris is forced to choose who to save.

Trivia

 * The title is possibly a play on the saying, "Walk, don't run".