This week's episode of The Flash, entitled "The Nuclear Man", marks the return of Firestorm, the villainous combination of Ronnie Raymond (Robbie Amell) and Martin Stein (Victor Garber). Today, The CW released a brief preview of tomorrow night's episode, where executive producer Andrew Kreisberg explains that fans will finally learn what exactly happened that lead to Firestorm's creation. As you may recall, Ronnie, the fiancee of Caitlin Snow (Danielle Panabaker), vaporized during the show's third episode, and the mystery behind how Robbie and Martin became Firestorm will be solved.

After Ronnie (guest star Robbie Amell) attacks a physicist (guest star Bill Dow), Barry and the team realize they need to go after Caitlin's (Danielle Panabaker) fiancé who is now a dangerous meta-human. They deduce the best way to track Ronnie is to find Dr. Martin Stein (guest star Victor Garber) who was working on a project called F.I.R.E.S.T.O.R.M. Barry (Grant Gustin) struggles to balance his duties as The Flash and his relationship with Linda Park (guest star Malese Jow), and Joe (Jesse L. Martin) enlists Cisco's (Carlos Valdes) help to reinvestigate the murder of Nora Allen (guest star Michelle Harrison), while General Eiling (Clancy Brown) returns to Central City.

In addition to this preview, The CW has also released a brief video where Grant Gustin answers questions from the fans, revealing which DC Comics superhero he would love to see appear on The Flash, his working relationship with Robbie Amell, and who the funniest actor is on the set, when the cameras aren't rolling. Take a look at these new videos before The Flash returns with "The Nuclear Man" tomorrow night at 8 PM ET on The CW.

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