Why Is Arrow's Laurel Lance Such A Major Player in Invasion?

When Arrow's hundredth episode airs tonight, Oliver Queen will finally be planning his wedding to Laurel Lance, as many fans had assumed he would eventually be when the series began.

Of course, that was before producers cast a young actress as an IT girl in a one-off season 1 episode and, after seeing her chemistry with series lead Stephen Amell, quickly promoted her to series regular.

In season 4, Laurel -- then acting as the vigilante known as the Black Canary -- was killed. Her death has affected a lot of change in the show's fifth season, as Oliver and his new Team Arrow have taken on a more deadly path to finding justice (Oliver thinking if he had killed Damien Darhk earlier in season 4 instead of putting him in prison, Laurel would still be alive) and Quentin Lance has found his way back to the bottle.

Since her passing, Laurel has been a frequent conversation topic, but hasn't -- even in the flashbacks -- been much of a physical presence on Arrow. So some fans might wonder why she looms so large in the episode that will air tonight.

Arrow executive producer Andrew Kreisberg said simply that she was an important part of the show, and always will be.

"She was the second lead on the show. Even though the show has evolved, Laurel was at the heart of it," Kreisberg told reporters at a screening of the Invasion! event on Monday. "She was Oliver's great love. She's Sara's sister. She's Lance's daughter. Watching those early episodes, so much of it revolved around her relationship with all of these characters. I watched the episode Vertigo where she's arguing with her father about how much Thea reminds her of Sara and how he should go easy on her. I'm sitting here listening to that thinking about, 'God, they didn't even know Sara was alive yet.' Even if we do get to two hundred episodes, Laurel will always be at the heart of the series and be such an important character. On top of that, Katie Cassidy will always be so important to us. We were so happy that she agreed to come back because she's part of the family, both behind the camera and in front of it."

Arrow, The Flash, and Supergirl will cross over with DC's Legends of Tomorrow this week in "Invasion!" a three-part crossover that starts in the final moments of an episode of Supergirl and then plays through the other series. Based on the 1988 comic book event miniseries Invasion! from Keith Giffen, Bill Mantlo, Todd McFarlane, Bart Sears, and the crossover -- with its marketing title "Heroes vs. Aliens" -- will closely track the plot of that storyline: aliens will be assembled by The Dominators and descend on Earth to bring an end to the "threat" of Earth's burgeoning metahuman community.

In the comics, the Dominion were secretly hoping to build their own super-race, and as a result created a handful of new superheroes in the course of the story.

In the TV version, rather than assembling a loose alliance of dangerous alien races, the Dominators will apparently be mind-controlling the aliens that join them. That's a revelation that suggests even the heroes might be forced to take one another (or at least the alien Supergirl) on at some point in the story.

NEXT: The Flash, Arrow, Supergirl, Legends of Tomorrow Crossover Trailer / First Look At The Dominators / Invasion! Crossover Promo Photos / Potential The Flash Spoiler Spotted On Set / Will Joe West Die During The Invasion! Crossover? / Enter To Win An Ultra-Rare Invasion! Crossover T-Shirt

Supergirl airs Mondays at 8 p.m. ET/PT; The Flash on Tuesdays at the same time; Arrowon Wednesdays and DC's Legends of Tomorrow on Thursdays. All four series air on The CW. The "Invasion!" crossover will air beginning November 28.