Triple-check your DVRs now, Arrow fans, because one of the CW series’ most resonant moments will be unfolding this week, as part of the annual crossover event with The Flash.

In this year’s two-night crossover event, titled “Legends of Today” and “Legends of Yesterday,” time-hopping baddie Vandal Savage (played by Casper Crump) sets his sights on Central City barista Kendra Saunders (aka Hawkgirl), whom Barry quick-quick-quickly hides away in pal Oliver Queen’s own hometown. As a result, much of the opening hour, on The Flash, takes place on Arrow‘s turf.

The flip side proves true when Part 2 arrives on Wednesday, relocating the action from Star City to Central City, resulting in what executive producer Greg Berlanti describes as a two-hour movie versus standalone episodes.

“Because Flash was in its first year last year, we tried with each to make it still feel like a Flash episode and still feel like an Arrow episode,” Berlanti explains to TVLine. “This year, we wanted to really blend them and really make it feel like, ‘What if you were to do a two-hour film with all these characters?’ That was our focus.”

That said, he reassures, “There are elements about The Flash that still feel like The Flash, but there’s stuff in each that reflect the other… and a real centralized story that goes throughout.”

The crossover will also uncork “probably the biggest emotional story we’ve ever done on Arrow,” Berlanti teases. (Whether it has to do with the latest reappearance by Oliver’s soon-to-be-named ex-girlfriend, played by Defiance‘s Anna Hopkins, Berlanti was mum, saying only that there is a moment “very akin” to last year’s crossover, where Oliver bumped into his old flame at Jitters, still unaware that she gave birth to and has been raising his child since their forced years-ago split.)

Whatever the nature of the aforementioned “emotional” twist, coming out of this week’s crossover event and heading unto the Dec. 9 midseason finale, Berlanti warns of a new test for Oliver and Felicity.

“What’s been really exciting about the show this year has been dramatizing their relationship, and how that’s informed his journey and her journey,” the prolific producer observes. “Like any relationship, there’s going to be ups and downs and surprises and pitfalls, and they have some real challenges going into the break. but that’s probably all I should say without getting in trouble with my cohorts!” (With reporting by Vlada Gelman)

What are your predictions, gentle readers, for the “biggest emotional story” to come, as well as the “challenges” ahead for Olicity?