'The Flash' and 'Supergirl' musical crossover releases in March

The long-awaited musical crossover between "The Flash" and "Supergirl" has been announced to release on March 21. This will be an episode of "The Flash" Season 3 although a lead-in will take place on March 20, during the end of an episode of "Supergirl" Season 2.

Dermott Downs has been confirmed to direct the episode, which is titled "Duet," Deadline reported. The episode will feature the singing performances of Grant Gustin, Melissa Benoist, Victor Garber, Jesse L. Martin, Carlos Valdes, John Barrowman and Jeremy Jordan.

Both Gustin and Benoist have starred in the musical series "Glee" and Martin is a renowned musical performer on Broadway. Garber also played the role of Jesus in the musical show "Godspell."

David Harewood and Chris Wood, both from "Supergirl," will also appear in non-singing guest roles for the episode. Their inclusion indicates there will be scenes set in Earth-38, the primary setting of "Supergirl," although the majority of the episode will be on Earth-1.

The addition of both Garber and Barrowman also hint at larger problems on the horizon. Both of their characters are part of the time-traveling series, "Legends of Tomorrow."

Previous rumors hinted that "Avengers" director Joss Whedon was in talks of directing the musical crossover but this did not come to pass, Comic Book reported. The task has instead fallen to Down, who directed the Season 1 finale of "The Flash" and both episodes featuring Gorilla Grodd.

The casting for the crossover episode's villain, the Music Meister, has yet to be announced. It is widely speculated this villain will curse both Supergirl (Benoist) and the Flash (Gustin), forcing them to sing instead of speak.

The "Supergirl" Season 2 midseason premiere airs this Jan. 23. The midseason premiere of "The Flash" Season 3 follows on Jan. 24. The official trailer for "The Flash" Season 3 midseason premiere can be viewed below: