"Arrow" airs on the CW in the US and Fox 8 in Australia. Facebook/Arrow

The third portion of the big CW crossover took place in an episode of “Arrow,” which was also titled “Invasion!” In it, the heroes -- including those from “Supergirl,” “The Flash” and “Legends Of Tomorrow” -- learn that some things just cannot be helped.

READ: The CW's upcoming crossover for 'Supergirl,' 'The Flash,' 'Arrow' and 'DC's Legends Of Tomorrow' will begin on Nov. 28

The episode begins with Oliver in a hallucination, where his ex-fiance Laurel (Katie Cassidy) is healthy and happy, much like he is. It all seems perfect until it is made clear that he, along with Thea (Willa Holland), John (David Ramsey), Ray (Brandon Routh) and Sara (Caity Lotz) are all in alien pods within the Dominators’ spacecraft.

Meanwhile, Cisco and the rest of the heroes work hurriedly to locate their teammates. Cisco touches one of Oliver’s bows to try and vibe their location, which shows a ship. The hallucination continues for the Dominators’ captives, who are starting to feel that the entire experience is unreal. In the dream, Oliver and Robert (Jamey Sheridan) are attacked by a thug and are correspondingly saved by an arrow-wielding vigilante -- John. This increases Oliver’s doubtfulness of the situation.

Oliver and John soon figure out that they are in a dream. Sarah joins in afterwards, and the three of them find that Ray and Thea are part of the dream as well. It is then clear that the Smoak Technologies building can get them out of their situation. Back on Earth, the team realizes that they need to head to Doctor Laura Washington, and Supergirl, Flash, Mr. Terrific and Wild Dog go on a chase for her. They successfully save Rene from the mecha-doctor.

Back on the ship, Thea does not want to leave the hallucination and suggests to Oliver that Kara (Melissa Benoist) and Flash (Grant Gustin) can keep the world safe -- that the dream is a reward for all their sacrifices. “I didn’t make those sacrifices for a reward,” Oliver tells her. “I did what I did because I thought it was right.”

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When the Dominators release a swarm of villains from their pasts, Oliver, John, Sara, Ray and Thea engage in combat. When the fight is over, they all get to say the goodbyes and apologies to their dream-family. The episode ends as the group steps into a portal.

“Supergirl,” “The Flash,” “Arrow” and “DC’s Legends Of Tomorrow” air on Mondays, Tuesdays, Wednesdays and Thursdays on US network The CW. Australian audiences can catch the same on Fox8.