Kara Danvers will gain a new ally in The Flash, who will help her take down Livewire and Silver Banshee in Supergirl episode 18. The Flash-Supergirl crossover is titled Worlds Finest, which will air on 28 March at 8pm EST on CBS Network.

Barry Allen (played by Grant Gustin in The CW show) from Central City will be stranded in Kara Danvers's universe following a rogue time-travel. The official synopsis for the episode reads as follows:

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, on Supergirl.

Click here to watch the episode live via CBS All Access. You can watch the episode online by clicking here (only in the US).

Grant Gustin spoke about the special episode and teased an "undeniable chemistry" between Kara and Barry. He told Entertainment Weekly, "We tease it a little bit. He's in town for a day so there's not really time for anything to actually develop, but there's undeniable chemistry, and they have a lot of things in common that they'll never have in common with anybody else, obviously. This is the first person with powers Barry [has met] who's using them for good. It happens to be somebody that he is attracted to, so it's cool."

Barry and Kara address the age-old question of who is faster – Supergirl or Flash. Executive producer Andrew Kreisberg revealed, "It's one of my favorite moments in this episode. It's close." Gustin hinted at a crossover race between Arrow, Supergirl, Flash and Legends Of Tomorrow. "That's in the cards at this point," teased Gustin, and added "The whole point of us doing this crossover is setting up the fact that maybe next year we can cross over all of them."