The Dominators are coming, and the Flash (Grant Gustin) will need all the help he can get. That’s why he’s recruiting Supergirl (Melissa Benoist), and he’s going all the way to her Earth, which was just officially designated Earth-38.

In a preview for “Invasion!,” this year’s annual crossover of DC Comics shows Arrow, The Flash, Legends of Tomorrow, and now Supergirl, Executive Producer Todd Helbing revealed that forming a team of “Super Friends” is Barry Allen’s idea, and that the Earth Kara Zor-El lives on is Earth-38.

“If they’re gonna fight aliens, he wants to use an alien,” Helbing explains, “So he goes to Earth-38 and gets Kara Zor-El, known as Supergirl.” Barry and Cisco (Carlos Valdez) did just that, appearing in this week’s mid-season finale of Supergirl that ties directly into the crossover.

In Season 2, the scope of the CW’s “Arrowverse” widened when Barry fought Zoom, a serial killer from the parallel Earth-2 and rescuing Jay Garrick (John Wesley Shipp), another Flash from Earth-3. Supergirl, which takes place in a different universe from The Flash, didn’t have an officially designated “Earth” until Helbing’s comments.

DC innovated the multiverse in comics in The Flash #123 in 1961, in which Barry Allen — the Flash of the Silver Age (1956-1970) — teams with Jay Garrick, the Flash of the Golden Age (1938-1955) in a story titled “Flash of Two Worlds.” Since DC’s New 52 comics reboot in 2011, there are 52 known “Earths” but not all of them have been fully explored. After this year’s Rebirth, the existence of these Earths remain unknown.

The Flash continues the crossover “Invasion!” this week on the CW.