The Flash S03-E17 (Duet )
Another one of those episodes that have been divided into two parts. Last one being The Attack on Gorilla City and The Attack on Central City, this episode will feature Supergirl. As it is quite literal from the title, Duet will be focussed on a tag-team effort by Barry and Supergirl. People are speculating that by then, Killer Frost will overpower Caitlin and she will rogue. She will try her best efforts to kill both of them but as said, it is nearly impossible to defeat the daughter of Krypton. She will help Barry out of the situation.