S1, E13: After Ronnie attacks a physicist (guest star Bill Dow), 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 (guest star Victor Garber) 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 (Carlos Valdes) help to reinvestigate the murder of Nora Allen (guest star Michelle Harrison). General Eiling returns to Central City.