This week, the Green Arrow (Stephen Amell) will be teaming up with Supergirl (Melissa Benoist), the Flash (Grant Gustin), and the Legends of Tomorrow crew as they defend Earth from an alien invasion. The CW crossover action goes into its third night on "Arrow," which also happens to be the show's 100th episode and will see Oliver waking up to a new reality.

In the episode titled "Invasion," Oliver wakes up to a life where he never got on The Queen's Gambit. His parents, Robert (guest star Jamey Sheridan) and Moira Queen (guest star Susanna Thompson) are alive and well. Laurel (Katie Cassidy) is alive and is his fiancée in this new reality, and their wedding is imminent. Everything seems to be perfect, but Oliver starts to notice small irregularities that make him question this new reality. Meanwhile, Felicity (Emily Bett Rickards) and the recruits take on a new threat with the help of the Flash and Supergirl.

In an interview with TVLine, Amell said that their 100th episode is "a radical departure from everything that we have ever done on the show."

"We utilize the fantastical elements that we bring in on 'The Flash' and carry through to 'Legends of Tomorrow' with the present-day storyline to create something...that is not a flashback [or] a flash-forward," the actor explained.

As Robert, Moira, and Laurel are miraculously alive in this episode, Oliver will get to have a sense of closure as he gets to say goodbye to his loved ones, since he wasn't given the chance to do so before they died.

Executive producer Marc Guggenheim teased that amid all the action in the four-way crossover, Wednesday night's episode will surely be an emotional one for viewers, as he previewed once scene which he said is Amell's finest performance to date.

"This one scene, that really has two moments within the one scene, will break your heart," said Guggenheim. "It's incredible."

"Arrow" airs Wednesdays at 8 p.m. ET on The CW.