Supergirl
World's Finest
S1 E18:
Supergirl gains a new ally when the lightning-fast super hero The Flash (guest star Grant Gustin) suddenly crosses over from an alternate universe and offers to help Kara battle Silver Banshee (aka Siobhan) and Livewire in exchange for her help in finding a way to return him home.
Starring
Grant Gustin, Jenna Dewan, Chris Vance