Barry asks Green Arrow for help when aliens attack Central City, but when they realize that won't be enough, they track down the Legends of Tomorrow so they can bring Supergirl in on the battle.
Cast: Grant Gustin, Candice Patton, Danielle Panabaker, Carlos Valdes, Keiynan Lonsdale, Tom Cavanagh, Jesse L. Martin, Stephen Amell, Melissa Benoist, Franz Drameh, Victor Garber, Greg Grunberg, Willa Holland, Caity Lotz, Dominic Purcell