Trailer for The Once and Future Flash, premiering April 25 The Flash won’t be back until Tuesday, April 25, but The CW has already released the trailer for the episode, titled “The Once and Future Flash,” which you can watch below. The Flash stars Grant Gustin as Barry Allen/The Flash, Candice Patton as Iris West, Danielle Panabaker as Caitlin Snow, Carlos Valdes as Cisco Ramo, Tom Cavanagh as Dr.