After Ronnie attacks a physicist, Barry and the team realize they need to go after Caitlin's fiancé who is now a dangerous meta-human. They deduce the best way to track Ronnie is to find Dr. Martin Stein who was working on a project called F.I.R.E.S.T.O.R.M. Barry struggles to balance his duties as The Flash and his relationship with Linda Park, and Joe enlists Cisco's help to reinvestigate the murder of Nora Allen. General Eiling returns to Central City. Glen Winter directed the episode written by Andrew Kreisberg & Katherine Walczak. Original airdate 2/10/2015.