Supergirl Melissa Benoist and The Flash Grant Gustin RTS1BODR
2017 Teen Choice Awards – Show – Los Angeles, California, U.S., 13/08/2017 - Melissa Benoist and Grant Gustin speak on stage. Benoist and Gustin both appear in the "Supergirl" season 3 "Crisis on Earth-X, Part 1" crossover episode. Reuters/Mike Blake

The "Supergirl" cast, which includes Melissa Benoist as Kara Danvers/Supergirl, Chyler Leigh as Alex Danvers and Mehcad Brooks as James Olsen/Guardian, was featured in the "Crisis on Earth-X, Part 1" episode. It aired in the US on Monday. It featured other characters from the DC multiverse including "The Flash" (Grant Gustin), "Arrow" (Stephen Amell) and White Canary (Caity Lotz) from "Legends of Tomorrow."

Spoiler alert: This article has Season 3 'Supergirl' spoilers. Read on if you want to know about the 'Supergirl' crossover episode.

In "Supergirl" season 3, episode 8, there's a parallel world called Earth-X ruled by Nazis. Unfortunately those loyal to Americans as well as other resistances were hunted down by Dark Arrow. Meanwhile, Alex and Kara travelled through a portal to arrive at Earth-1 and attend Barry Allen and Iris West's (Candice Patton) wedding.

Alex is still recovering from her break-up with Maggie (Floriana Lima). Thankfully, she met Sara Lance/White Canary at Barry's wedding to distract her from it and hopefully, help her move on. As for Kara, she joined Felicity (Emily Bett Rickards), Iris and Caitlin (Danielle Panabaker) for champagne and mani/pedis. Kara, being super strong, even broke the nail equipment. They have a bit of girly chit-chat, which included questions about Felicity and Oliver's relationship. The same went for Barry and Oliver, who wore their tuxedos while talking about vows and love.

Unfortunately, Iris and Barry's wedding was interrupted by invaders from Earth-X, which were led by Dark Arrow and Overgirl, his wife from Krypton. Overgirl was injured by Kara, forcing the Nazis to retreat. They were also able to capture Prometheus (Colin Donnell), who is Dark Arrow's partner.

Elsewhere, the villainous group, which also includes a yellow speedster, regrouped and talked about their next move. At the end of the episode, the yellow speedster was revealed to be Dark Flash/Reverse-Flash aka, Eobard Thawne as Dr. Harry Wells (Tom Cavanagh). As for the dark archer, he is a doppelgänger of Arrow/Oliver Queen. Plus, Overgirl is a doppelgänger of Kara. Larry Teng directed this episode, which was written by Marc Guggenheim and Andrew Kreisberg (story). The teleplay was written by Jessica Queller and Robert Rovner.

'Supergirl' episodes

"Supergirl 3" airs in Australia on Fox 8. It also airs in the US at 8-9 pm ET/PT on The CW every Mondays. "Crisis on Earth-X, Part 1" will air in Australia on Thursday at 3 am. The episodes after it are "Reign" and "Legion of Super-Heroes." "Reign" airs in the US on Dec. 4 and in Australia on Dec. 5. "Legion of Super-Heroes" will air in the US on Jan. 15, 2018. Stay tuned for more spoilers about the Girl of Steel, aka Superman's cousin, and her friends.

Watch the 'Supergirl' crossover episode 'Crisis on Earth-X, Part 1' videos below: