In this all-new clip from "The Flash," Barry Allen (Grant Gustin) and Caitlin Snow (Danielle Panabaker) confront Firestorm with a figure from Martin Stein's (Victor Garber) past.

Tonight's episode, "The Nuclear Man," brings Firestorm into the fold as Barry and Caitlin work to separate Stein from Ronnie Raymond (Robbie Amell) in order to restore Caitlin's fiance. The clip finds Barry, Caitlin and Clarissa -- a figure from Stein's past -- confronting Firestorm once and for all to try to convince him to work with STAR Labs.

Check out the full clip below:

Tonight's episode of "The Flash," "The Nuclear Man," airs at 8 p.m. EST on the CW.