SPOILER ALERT: Do not read if you have not yet watched “Crisis on Infinite Earths: Part One,” the premiere of the 2019 “Arrowverse” crossover.

Bringing “Crisis on Infinite Earths” to the small screen has been years in the making, so it’s no surprise the epic five-part crossover kicked off with plenty of action on Sunday night with “Supergirl.”

Following a quick refresher of the multiverses (including an appearance from Burt Ward), the cameras honed in on Earth-38 where a dishevelled Wil Wheaton made a cameo as a placard-carrying man alerting everyone to the impeding doomsday. Enter a pants-wearing Kara aka the titular Supergirl (Melissa Benoist) — complete with bangs — to stop the immediate crisis of an angry and oversized lizard, but there were bigger things at play.

The resident superheroes of Earth-38 quickly realized the shocks hitting the planet weren’t actually earthquakes but a wave of antimatter on its way to take them out. But first it was going to bring down Argo City, where Clark (Tyler Hoechlin) and Lois (Bitsie Tulloch) were learning to deal with dirty diapers. Kara’s mom (Erica Durance) was also hanging out in Argo, and when Kara warned them that things were about to go down they realized there was only one escape pod. And so, baby Jonathan was doled out the same fate as his father and sent to Earth before Argo City was obliterated in front of Kara’s eyes.

Meanwhile, Lyla (Audrey Marie Anderson) fully embraced her Harbinger alter-ego and summoned the rest of the DC superheroes, beginning with Oliver aka Green Arrow (Stephen Amell) and Mia (Katherine McNamara), who were bonding on that infamous island; Barry aka The Flash (Grant Gustin); Kate aka Batwoman (Ruby Rose); Ray aka The Atom (Brandon Routh) and Sara aka White Canary (Caity Lotz). They all appeared on Earth-38 to help Kara and co. keep the antimatter at bay, while they figured out how to either defeat it or get all of the citizens as far away from the doomed planet as possible.

In a twist of events, Clark and Lois also appeared, only to reveal that The Monitor (LaMonica Garrett) saved them but was unable to help Kara’s mom. Safe for now, Lois took off after baby Johnathan, whose pod was redirected to Earth-16 in 2046, along with Brainy (Jesse Rath) and Sara.

With the gang back together and a newly surfaced quantum tower ready to help ward off the energy, the clock was ticking as Alex (Chyler Leigh) turned to the only person she knew could help create a portal large enough to send people to Earth-1: Lena Luthor (Katie McGrath). Although the duo clearly need some couples therapy after all of the bad blood between them, Lena agreed to help, eventually pulling out the math (with Alex’s help) to allow the portal to open.

Meanwhile over on Earth-16, that iteration of Ollie intercepted baby Jonathan, only to stumble when he came across Sara. In his mind his former flame was still dead, and seeing her again led to a moment of truth and reconcile between the torn-up Oliver and a tearful Sara.

Concurrently, Oliver prepped for what would turn out to be his final battle by gifting his daughter her own Green Arrow outfit, and the duo took up arms inside the tower to ward off a Dementor-like army of flying ghosties intent on breaching the walls. As the antimatter gained traction Clark and Kara used their heat vision to re-power the solar panels and buy some more time, while below the rest of the team worked in tandem during the highly choreographed fight scene.

It all culminated in a tense retreat as The Monitor reappeared to pull the heroes out one-by-one, but Oliver refused to go. The Monitor was rendered helpless after receiving an arrow of his own, while Ollie proved to audiences everywhere just how far he’s come as a hero. He slung arrows as fast as he could at the never-ending army, until his own quiver emptied and he succumbed to the ghosts. The entire world imploded and for a brief commercial break Oliver’s fate was up in the air.

Back on Earth-1 it was quickly revealed that The Monitor had managed to move Ollie’s body to rejoin the rest of the team, but the damage had been done. In front of a guilt-ridden Wells (Tom Cavanagh), who had set this entire thing in motions, Oliver told Barry and Kara he had sacrificed himself so they could rise up and defend the universe, and he once again begged Mia to find her mother. The Monitor estimated this sacrifice had saved approximately one billion lives, an even more significant number given that only three-something billion had made it safely to Earth-1 before Earth-38 fell.

Yet, even he couldn’t foreshadow Oliver’s death the way it went down, and as the heroes were left standing looked around at each other in grief over Ollie’s death it was Wells’ words that closed off the first part of the crossover: “Everything we know, everything there is, and everything there ever was, is doomed.”

Surely the DC heroes have something to say about that when “Crisis on Infinite Earths” continues Monday, Dec. 9 at 8 p.m. on the CW.