The first full-length trailer for the upcoming crossover between "The Flash" and "Supergirl" has been released, giving fans their first good look at Supergirl (Melissa Benoist) and The Flash (Grant Gustin) teaming together.

According to the synopsis from TV Line, The Flash will be arriving in Supergirl's universe. Fans speculate that this may be an effect of The Flash's attempt to discover a means of traveling to Earth-2 now that the wormhole they used to travel through closed in "The Flash" season 2 episode 15. 

Instead of arriving on Earth-2, however, he ends up in an alternate universe where Supergirl and her cousin Superman are the main superheroes at play. The synopsis reveals that he will be aiding Supergirl in taking down two supervillains who attack simultaneously, and in return, Supergirl will help him find a way back to his universe.

The villains mentioned are Banshee, the alter ego of Siobhan Smythe (Italia Ricci), and Livewire (Brit Morgan). 

"It is the most joyous occasion to have these two superheroes together in the same place, at the same time, working together," stated Benoist. "They are both so happy with their powers and what they get to do and how they get to help people, that it's kind of infectious and exciting and upbeat and happy."

Scenes in the trailer play on this concept as they reveal The Flash and Supergirl racing against each other, attempting to put to rest the common argument amongst comic book fans on which superhero is the fastest.

In the original comic books, it has been perceived that The Flash is only slightly faster than either Supergirl or Superman, although this has been subject to change from time to time. In the trailer, it is hinted that The Flash allows Supergirl to win. 

The Flash is also seen interacting with many of Supergirl's allies and it is likely that they will be aware of his secret identity. Cat Grant (Calista Flockhart) is also shown bearing interest in The Flash and it is likely she will want to write up a story about him before The Daily Planet does. 

The crossover between "Supergirl" and "The Flash" will air on the "Supergirl" time slot as episode 18, "Worlds Finest." The episode will air on CBS this Monday, March 28.