A lot has happened in the lead-up to the Arrowverse’s 2019-2020 crossover, Crisis on Infinite Earths, so a recap may be in order. The Arrowverse has been building up to Crisis on Infinite Earths for quite a while, particularly in the current seasons of Arrow and The Flash, which have been centered around the coming conflict.

Crisis on Infinite Earths is perhaps the most important DC Comics miniseries ever published, so it makes sense that the 1985-1986 comic served as the basis for the Arrowverse’s biggest crossover ever. In the comics, the heroes of the DC Universe had to band together to protect the multiverse from an evil entity known as the Anti-Monitor, who had orchestrated a plan to erase all of existence. Crisis on Infinite Earths culminated in a dramatic shift for DC Comics that created a singular timeline for all of its characters.

This is the story that the Arrowverse is adapting for this year’s crossover, but how it will all shake out in the end remains to be seen. The Arrowverse’s version of the story will be told across episodes of Supergirl, Batwoman, The Flash, Legends of Tomorrow, and Arrow, and will feature a massive cast of characters from both inside and outside the Arrowverse. Unlike with past crossovers, the Arrowverse has been laying the groundwork for Crisis on Infinite Earths for a long time, so viewers who are just jumping on for “Crisis” may have missed out on a lot of the set-up for the story. Here’s what you need to know before Crisis on Infinite Earths begins.

How Arrowverse Has Teased Crisis On Infinite Earths Since The Flash Season 1

In the pilot episode of The Flash, a newspaper from the future revealed that Barry Allen (Grant Gustin) would vanish in an unspecified Crisis on Infinite Earths that would take place in 2024 (this date was later moved up to 2019). This means that the Arrowverse has been teasing Crisis on Infinite Earths as early as 2014. Crisis on Infinite Earths was officially confirmed to be the next event at the conclusion of last year’s Elseworlds.

At the end of Elseworlds, it was confirmed by a cosmic being called the Monitor (LaMonica Garrett) that a Crisis was on the way, and that the universe would never be the same. The Arrowverse provided further setup at the end of Arrow season 7 when the Monitor recruited Oliver and told him that he would be needed to save the multiverse. Team Flash received a similar warning at the beginning of The Flash season 6. Barry was told that waves of antimatter would consume entire worlds until nothing was left.

The Key Moments of Crisis on Infinite Earth So Far

Crisis on Infinite Earths has yet to officially kick off, but certain events connected to the story have already occurred. Earth-90, which is the home of John Wesley Shipp’s Barry Allen and the 1990 The Flash TV series, was destroyed in Elseworlds by the Monitor, leaving him as its sole survivor. The Monitor was testing the worthiness of its planet’s heroes. Apparently, the Monitor was looking for an Earth that would have heroes that he could use to deal with the Crisis.

In the Arrow season 8 premiere, Earth-2 was wiped out by a wave of anti-matter, seemingly killing off a multitude of characters, two of which being from The Flash. Earth-2 was the first known causality of Crisis on Infinite Earths, and the first time that Oliver (Stephen Amell) and Team Arrow were able to see with their own eyes what they were up against.

Throughout Arrow season 8, the Monitor has been sending Oliver on a number of errands, with Oliver oblivious to the true purpose behind his actions. All Oliver knew was that he was gathering parts for a weapon. In the last episode before “Crisis”, the weapon was complete, and it was revealed to be a device that transformed the head of A.R.G.U.S., Lyla Michaels (Audrey Marie Anderson), into the Harbinger, whose main responsibility appears to be recruiting heroes for the Crisis. It would seem that Harbinger is the key to Monitor’s plan to survive the Crisis.

The Harbinger may be just one of several pieces on the board for the Monitor. On this season of The Flash, viewers met a new version of Tom Cavanagh’s Harrison Wells, Nash Wells. Nash is a multiverse explorer with a plan to hunt down and kill the Monitor. After uncovering a wall with mysterious symbols, Nash was consumed by a bright light that may have transformed him into Cavanagh’s Crisis on Infinite Earths character, Pariah. The importance of this scene is indicated by the fact that it was shown at the end of four different Arrowverse shows, Batwoman, Supergirl, The Flash, and Arrow.

Oliver Queen & Barry Allen Are Both Supposed To Die In Crisis

The protagonists of the first two Arrowverse shows are both fated to die in Crisis on Infinite Earths. In separate conversations, Oliver and Barry have been told by the Monitor that their deaths are necessary for the survival of the multiverse. Both characters and their supporting casts have spent their respective seasons coming to terms with their fates. Due to a surprising time travel twist, Oliver was able to connect with his adult children from the future, while Barry worked toward making Cisco (Carlos Valdez) to become the new leader of Team Flash and preparing Elongated Man (Hartley Sawyer) to serve as the new protector of Central City. The Monitor hasn’t given specifics on how either of them will die.

Which Key Arrowverse Characters Are Missing From Crisis?

Considering the sheer size of the cast of Crisis on Infinite Earths, there aren’t many major Arrowverse characters missing from the event. The biggest omission is Oliver’s wife, Felicity (Emily Bett Rickards), who exited the series at the end of season 7. Even so, she will at least be back for Arrow’s series finale. Another character skipping the Crisis is Kid Flash (Keiynan Lonsdale). Some “Crisis” characters haven’t even been announced, so there isn’t a clear picture of what the full cast looks like just yet.

More: Monitor Recruits Major Supergirl Hero For Crisis On Infinite Earths