Supergirl/Flash Crossover Poster & Plot

As you may know The Flash is crossing over with Supergirl on the Monday March 28th episode on CBS.  CBS has released this very cool poster for the episode as well as the plot synopsis for the episode titled, "World's Finest":  “Kara gains a new ally when the lightning-fast superhero The Flash (Grant Gustin) suddenly appears from an alternate universe and helps Kara battle Siobhan, aka Silver Banshee, and Livewire in exchange for her help in finding a way to return him home.”  The Flash (Grant Gustin) guest stars when he crosses worlds with Supergirl (Mellisa Benoist) later this month!