The opening scene of The Dark Knight Returns uses a sporting event – a car race – to describe in symbols all the major events of the entire story. The car (world) goes out of control and the driver (Batman) is the only one who can control it. It looks like he dies, but he escapes certain death.
Batman #21 begins with a similar sporting event, whose actual event (a hockey player dies in a fistfight) is probably not as significant as the symbolism that Tom King – almost certainly working under the vision of Geoff Johns – provides. The contest is between Metropolis and Gotham City with "two heavy hitters" confronting one another, and if you need to be told who those two cities represent, you've probably never read a comic book. If you still don't get it, the Metropolis team's colors are red and blue while the Gotham team wears black and yellow. And if you're wondering what the outcome will be, the Gotham team is named after a deadly weapon and the Metropolis team is named after an extinct species. And if this fight is meant to be prophetic, the Batman surrogate beats and literally kills the Superman surrogate.
We're not the only ones to get the symbolism. Saturn Girl, in Arkham Asylum, has privileged knowledge about the future, and she lets it slip that she's not talking about more than what we see in the hockey game when she says "They're going to kill him." Not the one person – "he" – that we see on television, but "they." Maybe the hockey game is tragic in its own right, but Saturn Girl sees something else – "Superman won't come. Our friends will die. The Legion will die." If the hockey fight represents a superhero calamity to follow, Superman will somehow be sidelined by Batman, and will therefore be unable to save the lives of the Legion and their friends (present-day superheroes?). Remember, Geoff Johns used Saturn Girl's fellow Legionnaire Star Boy in an almost identical fashion after Infinite Crisis, with the now Star Man taking over the role of the eponymous member of the Justice Society and providing ample quantities of foreshadowing along with mental illness.
These events are the kick-off to the big DCU / Watchmen crossover event that's coming, as symbolized by the hockey commentator's phrasing: "We're down to the final minute here, folks." (FYI, hockey overtime isn't one minute.) And then, "Here we go."
But there are important plot points, too. Reverse Flash remembers the Flashpoint universe in which Thomas Wayne was Batman, and he speaks to the no-longer-living Thomas Wayne in a mocking tone, enjoying how the elder Wayne died and how it hurts both Waynes when Thawne rips up the letter that Thomas sent to Bruce. The carrier of that message (a la the Roman god Mercury, who inspired the Flashes in many ways) was Barry Allen, so it's very appropriate that the message is destroyed by the Reverse Flash, un-doing something that the Flash did.
Like Saturn Girl, the Reverse Flash has information that allows for a very Johns-ian lookahead, and he tells us that some power resurrected him. This is undoubtedly for a reason and there are only so many possibilities. In this issue, the Reverse Flash beats the tar out of Batman, rips up the note from the Flashpoint universe, vanishes into a blue glow, returns speaking of God, and dies. And, I would caution the reader: We don't know if they actually occurred in the simple linear fashion that we thought we saw. If we take the events on their surface form, this Reverse Flash has come to this world from the world of Flashpoint, which makes him as well as the letter alien objects in this timeline, and if his purpose was to destroy the letter, and whoever revived him wanted to eliminate the connection between this Earth and the Flashpoint timeline then it's logical that Thawne needed to die after destroying the letter. It appears as though he did, but maybe something trickier happened. Reverse Flash was thoroughly splashed with blood before he vanishes, but when he returns, there is no sign of blood. Maybe he went through a physical experience that removed the blood (along with a fair bit of his own flesh), but maybe the timeline is trickier than it seemed, and the Reverse Flash who returned might be from a very different moment in his timeline than just a little bit after his fight with Batman. Otherwise, why make it so complicated as to have him disappear and reappear, instead of just burst into a blue flame and die? Note also that he disappears when the button is in his hand, but he returns with no apparent trace of it. Perhaps it just got lost in the violence of the moment, but given that the story is named after this object, it seems odd for it to be misplaced as a small detail.
There's another important reference/recurrence here: In Crisis on Infinite Earths, Batman sees a dying version of the Flash (Barry Allen) who is jumping in and out of time from later in the story. A dying Reverse Flash also appears before Batman here, which makes for the second reference to COIE, the other being Psycho Pirate's mask. All told, we have four objects from other timelines: The button, the letter, Thawne, and the mask. At the end of this story, two or three of these objects have been eliminated, leaving possibly just one. Clearly, someone is trying to eliminate connections between worlds, or at least certain connections. Perhaps the person pulling the strings is Dr. Manhattan, perhaps Mr. Oz (who also took Tim Drake "off the board" for presenting a similar threat). Perhaps the multiple, quick actions in this story were taken by more than one player, with someone wanting to get rid of the letter and someone else wanting to get rid of the button.
At midnight, all the agents and superhuman crew go out and round up everyone who knows more than they do.