Arrow's Marc Guggenheim: The 100th Episode is "Looking Back to Look Forward"

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.

"I think honestly where my brain goes is, the original goal was just to make a really great pilot," executive producer Marc Guggenheim told ComicBook.com. "The idea that we've made I think a really good pilot plus 99 episodes beyond that is mind-boggling."

The series since then has been a tale of lies told and truths untold, of the gradual unfolding of the truth behind Oliver Queen's "five years in hell," and the fifth season is culminating with Team Arrow squaring off against Prometheus, a character who apparently has roots in the first season of Arrow, which the show has been overtly referencing throughout the first half of the season, and which the show will revisit in an alternate reality tonight and in a flashback during next week's midseason finale.

Going "back to their roots" and exploring different takes on the first season? For a lot of casual viewers, that feels a bit like a reboot, and Guggenheim doesn't disagree -- though he seems to think the opinion is a little overstated.

"I think every year to a certain extent feels like a reboot. every year we always try to give the show a new identity," Guggenheim said. "We always said we want you to be able to channel surf or look at an episode randomly and immediately identify what season it is. Since the show does reinvent itself every year, I will say that this year, becuase it was the fifth season, the final year of the flashbacks, the hundredth episode season, we looked a lot at what the show's legacy was, what Oliver's legacy was, and we looked al ot at the previous four season. In terms of that, there was a sense of looking back to look forward that felt appropriate given all the things that the fifth season represents. I love the fact that we have over four seasons' worth of history and character development and episodes to draw upon."

And as for the big mystery everyone will be asking about: after letting his hair grow for four years, then cutting it last season, how will Oliver get his unruly head of "island hair" back in time to be rescued at the end of the season/in the pilot?

"Well, let me tell you -- I know everyone's going to be asking that question," Guggenheim laughed, and then, motioning to his bald head, he said, "I know I'm probably the least qualified to answer it. And I know that we're not going to do a crazy time jump anytime this season. All that said, we have a very good answer for it."

Some things, alas, have to remain a mystery for now...!

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.