The premise is simple: after changing the past to save his mom, Barry Allen gets stuck in 2013 when Zod's invasion is happening... Barry is an unapologetic Snyder's Superman fan 😅 and knows he needs to find Kal to stop the kryptonians. Unfortunately, Superman can't be found because altering the past broke the timeline messing up all the heroes' lives and none of them is around for one reason or another... not even the captive Kara knows where the cousin she has swear to protect is (Kara/Supergirl is more a MacGuffin in the film, but Sasha Calle is good with what she has to work with in her limited screentime).
The group that Barry put together has to fill for Clark's absence to fight the evil kryptonians but turn out you really need Superman to do Superman's job, and, this time, without Kal being around, Zod wins. The basic concept is that infinite attempts can be made to change the battle's outcome, but all the possible variations will always end up with Zod obtaining the codex and terraforming Earth (it's a fixed point)... so the price for Barry saving his mom was dooming the human race to be wiped out.
As I said in the past elsewhere, it's the MoS event gone wrong... if, in the original Snyder's films, the kryptonians' invasion was the catalyst that kickstart the age of heroes and birth the DCEU, this is its death: there's no Superman to prevent Earth to be terraformed, and no JL to come out the shadows after him.
The only way for Barry to stop Zod and his army is to make the ultimate sacrifice and let his mother die again to restore the DCEU events as they originally were. At the end, you can assume Superman got his job done (again) if Barry comes back in present day and the human race still exists. Sadly, the epilogue doesn't have a last appearance of Henry Cavill (there are only few brief CGI cameos of him left in the film) or a closure for the DCEU or the film itself (supporting characters like Calle, Keaton and Affleck vanish completely but you shouldn't even care) and you have to fill the blanks by yourself. A very unsatisfying way to conclude a story and universe, imo.
Beside DCEU Superman's "spiritual presence" across the film, we see a glimpse of the "SuperVerse" over there when the multiverse is collapsing during the final act: in a Black & White world, George Reeves appears with his own Jay Garrick; elsewhere Chris Reeve and Helen Slater stand side by side after flying; and a long haired Nic Cage is fighting a giant mechanical spider (yes, Nic finally got his chance to be Superman!). It seems a small tease of the original Crisis project that was planned under the old regime (imagine if we could've seen Cage and Cavill's Supermen teaming up to save the multiverse in a Crisis event 😆 ). |