Spoilers incoming,

Things start way before the final battle that we saw in Earth-2 #1. Basically Alan wanted to make the world better and financed Sloan's research of boom tubes. Sam is against it and eventually leaks that information to someone.
We see Sloan observing Terrific. Nice callback to DC Universe Presents #0.
The 9th dimensions. Steppenwolf, together with Fury, is preparing for invasion. They have Mister Miracle and Barda captured (remember when we first saw them they were running away and Fury was chasing them). Grundy is with them, supposedly avatars of this world were easy to seduce. Is this real reason why Green did nothing during the war and not the bullcrap that it said when Alan asked about it way back in Earth-2 #2 I think?
Interesting bit, Steppenwolf seams to be more worried about Wonder Woman than Superman. It works in a sense that Wonder Woman was first superhero on Earth-2, but maybe she was stronger than her Earth-0 version too?
Traveler, who is probably Sloan, gave up Earth-2 to save his own world. Works nicely with his constant "I'll do everything to save the world" motivation that he was going with.
So, Sam leaked information to Accountable (Jimmy Olsen) but before he can do anything with it his "base" is invaded by Batman and Robin.
Before Jimmy has enough time to show that Parademons are visible in the portal that Sloan is working on he makes a mistake and calls Batman by his name. Bruce not wanting to risk exposure immediately knocks him out and as we know from Earth-2 he gets put into stasis and never able to warn of whats coming.
We see Lois at her work when invasion starts with Parademons invading biggest cities. This is when Lois dies.
Catowman dies in a battle against Parademons. Amazons are also invaded.
World Army is formed to repeal attacks but unknowing that Sloan is responsible for this in the first place they enlist his help which ends up with fire pits being opened as seen in Earth-2 #0.
Trinity, Powergil and Robin meet in the Arkham prison (where stasis were). Batman interrogated Olsen, he was the one to give them the virus that they used to end the war.
Its interesting. It is mentioned that WW and Batman were killed, but that Superman was taken while Robin and Supergirl were blown away into that forgettable WF series.
Graysons survived the war. We see some panels with Jay and Mercury.
Sloan gets mad that invasion was stopped and finds out that Sam leaked the information about his project.
We see that Sloan was responsible for Aquawomans capture too and helped with Red Tornado becoming Lois. And train goes boom with Alan and Sam.
We see a panel were Atom looses his arm, but art kinda fails here and it looks like Superman took his head, not arm.
Eventually wee see recent event of Earth-2 series and Bizzaro Superman crumbling.
So whats new?
Flash is running around the world gathering intel on how various strongholds are doing since communication systems are still not fully functional.
He runs past Grayson who is in some survivors camp. Glad to see that Graysons survived second war too. Barbara was police officer while Dick was a journalist.
Helena and Kara are back, they observe ruined cities and see on of the still functional TVs new Batman and Superman.
Batman, Superman, Green Lantern and Red Tornado are helping out civilians. Batman thinks its not over and that they need to track down Bedlam. Eventually Kara and Helena get to them.
Khan gets into standard argument with the Government. They want to rebuild while he wants to hunt down Bedlam and seal fire pits.
There is also Oliver Queen that they are searching for, but its implied that he can't be trusted.
Terrific and Sloan built something for Bedlam, he is going to reward them with death while Miracle is chained, he is waiting for right moment to escape. We get confirmation, Steppenwolf was Fury's father and someone calls from Apokolips as I understood.
Hawkgirl is looking for Fate, he is still a bit crazy. He mentions a visitor coming from beyond the veil.
Kara and Helena were about to start fight with Batman and Superman but fortunately they start talking.
As they hug and are happy about stuff fire pits explode and, I assume, horseman enter the scene.

Overall I liked it. Weakest part was art since we got multiple artists and some pages were lacking or didn't work as well. I think it decently recaps of what came before this and sets up the stage for whats to come. I especially liked that not much was changed, only part that feels completely new and kinda retcon is Sloan being not from Earth-2.