Actor Grant Gustin (The Flash) revealed on Instagram the title and credits for 'The Flash' and 'Supergirl' crossover episode, titled “Worlds Finest.”  Story by Greg Berlanti. Teleplay by Andrew Kreisberg & Micheal Grassi.  ‘Supergirl’ airs Mondays at 8 PM ET on CBS. ‘The Flash’ airs Tuesdays at 8 PM ET on The CW.