It looks like the ability of The Flash to crossover into alternate earths extends into real-life. Greg Berlanti and Andrew Kreisberg, the producers of the show of the Fastest Man Alive, have announced that he will make a guest appearance in Supergirl, their other superhero series. While crossovers have happened before as far as the Flash is concerned, this team-up is a coup because Supergirl and the Flash are airing on two different and competing networks.

That has not stopped fans for asking for a team-up of the two young heroes. Unlike Stephen Amell's dark Green Arrow, Flash (played by Grant Gustin) and Supergirl (Melissa Benoist) are happier, younger super-powered beings who infuse a sense of fun and lightness in their non-stop saving of the world.

According to Io9, it's the Flash who will make a guest appearance on Supergirl on March 28. This happened only because the powers-that-be all put aside their differences to give their fans a special treat: CW, the Flash's network; CBS, which airs Supergirl; and DC Comics and Warner Brothers which own the rights to both properties.

The joint statement made by the two producers to Entertainment Weekly was positively gushing: "We want to thank Grant Gustin for making the time to come visit (on top of his already immense workload) and all of the folks at CBS, The CW, Warner Bros., and DC for working this out. And finally thanks to the fans and journalists who have kept asking for this to happen. It is our pleasure and hope to create an episode worthy of everyone's enthusiasm and support." 

Comic Book Resources posted its theory on how the crossover will happen onscreen, based on an Instagram posting of the title of the Supergirl March 28 episode: "Worlds End." The absence of an apostrophe hints that Barry Allen will use his speed force to cross over to alternate Earths, among them the Earth where Supergirl lives and is fighting super-villains.  

Supergirl airs Mondays at 8 p.m. ET on CBS, while The Flash airs Tuesdays at 8 p.m. ET on The CW.