Act 1:
the movie opens with the Justice League fighting the Legion of Doom (Lex, Deathstroke, Black Manta, Clayface & multiple henchmen). As Lex (Jesse Eisenberg) is close to ramming a Kryptonite dagger into Clark's (Henry Cavill) chest Batman (Ben Affleck) jumps between the two but while concentrating on Lex is killed by Deathstroke (Joe Manganiello). They might have "won" the battle but Bruce is dead, Barry (Ezra Miller) blames himself and leaves. He has a conversation with his father (Billy Crudup) about losing his mother (Amy Acker) until Henry says something along the lines of "if I could turn back time". Barry is desperate and hurt, so much so in fact that he decides to change time. We see a training montage of him gathering his speed. After it we meet him along with Cyborg (Ray Fisher) who warns Barry of the butterfly effect, Barry shrugs it off and denotes this as a "test run". A test run to the day of his mothers death. Barry speeds off, over flashes of Bruce's death we see him get angry and he starts to travel through time.
Act 2:
Back in the early 90s Barry finds Nora and Henry living a happy life. Meanwhile we see an ad for Wayne Enterprises where a young Bruce Wayne (Daniel Gillies) is shown, reminding Barry of why he even came. As he prepares to speed back he encounters something: two speedsters, apparently fighting, travel to this exact point in time and straight into the Allen household. Intrigued he follows and wittnesses the Reverse Flash (at this point unknown to Barry) attempting to kill Nora while an older version of himself is beaten unconcious. Barry prevents this however and the two engage into a fight that ends with Barry dragging RF (Richard E. Grant) back into his time. Back in "his" time Barry and RF have their first conversation that reveals RFs longstanding hatred for Barry making him kill Nora, before RF dissapears though he notes that "if Barry grew up in a world without Nora he broke his own reality by stopping RF from killing her".
Act 3:
A severly beaten Barry decides to check if RFs prediction was true and speeds to Gotham where he hears about the cruelty of "The Batman", thinking he inadvertetly saved Bruce he decides to meet him. Upon witnessing Batman brutally beating one of Joker's thugs however Barry finds out that this is in fact Thomas Wayne (Jeffrey Dean Morgan) and that instead of his parents Bruce died in that reality. Upon realizing that Barry changed reality so much that his son had died Batman attempts to kill Barry but is stopped by him as Barry promises to "make things right". Barry can't replicate his speed however as it appears that his powers have been severly weakened by his previous time travel. In the meantime Reverse Flash assembles Lex Luthor Sr. (Billy Zane), Black Manta (Yahya Abdul-Mateen II), Clayface (Timothy Hutton), Cheetah (Summer Glau) and The Joker (Lauren Cohan) and warns them that "their reality" is threatened by Barry.
To find out how to regain his speed Barry and Thomas seek the aid of Superman, only to find out that he's only Clark Kent in this world and has never chosen to become a hero. Barry gives Clark a speech about who Superman is but Clark walks away from the conversation. Instead they travel to Thymseciria to seek Wonder Woman's (Gal Gadot) aid. However it turns out that she leads the Amazonian armies in a war against the Atlanteans, led by Aquaman (Jason Momoa). Barry attempts to convince the two that in their original timeline they were friends and would have died for each other but both denote this and instead opt to remain staying at war.
Act 4:
Hellbent on "getting Barry this time" Reverse Flash finds and abducts Nora only to have her threatened on live TV by the Joker, attempting to lure Barry out. Thomas, Diana and Arthur tell Barry not to go as this is a trap. A discouraged Barry says that he has to go, seeing as this world has no more heroes left.
Barry faces the Reverse Flash's Legion on his own, finding out in the proccess what motivates RF: "In order to become the hero this universe needs you to be you need to feel the pain we felt, you need to be aware that you're no god" RF sprouts as he and his soldiers tower over Barry revealing that in his time RF is actually an admirer and the Flash, not his enemy. As the fight takes place in the heart of central city it is all witnessed by television cameras, we see Clark watch the footage on his Television. With Barry beaten down RF goes to Nora and attempts to kill her in front of Barry's eyes. "You need to feel true despair to save us all" he says as he's hit with a Batarang, Thomas appears and helps Barry up "No time to stay down, you still need to save my son" he says. Shortly before Clayface (also part of RFs Legion) can hit both he's hit Aquamans spear as both Arthur and Diana appear. It turns into another iteration of the fight between the Justice League and the Legion of Doom. The fight commences and as things look bleak even Superman joins in, giving Barry hope once more. Through this he seems to unlock his timetravel powers again and engages RF in battle. It's Joker vs Batman (with emotional speeches like "our son lives in another time"), Black Manta vs. Aquaman, Clayface vs. Superman, Cheetah vs. Wonder Woman and Reverse Flash vs. Flash.
As the fight ends RF is beaten and spared by Barry but as RF gets back up to kill Nora he's shot to the head by Batman.
Act 5:
The newly formed League says their goodbyes to Barry, who prepares to make his way back to stop himself from saving Nora. Thomas tells Barry that even if he didn't manage to save Bruce he knows that his son would be proud of him regardless.
Barry speeds into the past and stops himself from changing the past, saying that his personal tragedy makes Barry who he is and that changing the past will only fracture the world as it is. Barry returns to his time and is greeted by Cyborg who asks him "if it worked". Barry says that it did but not the way he thought it would.
The League is united at Bruce's funeral along with Alfred (Jeremy Irons), Lois Lane (Amy Adams), Comissioner Gordon (J.K. Simmons) and Barbara Gordon (Allison Brie) while a mysterious dark haired woman (Kate Beckinsale) stands in the back. Barry talks to Bruce's grave and puts a letter from Thomas on his gravestone.
Mid-Credits Scene:
We see four persian looking men carry Bruce's body in a ceremonial manner. The camera zooms in from behind on a man overseeing the procedure (played by Oded Fehr). "When you came to me, you were but a weak boy but the day you defied me you became my best creation" the man says as Bruce is lowered into a pool of green ooze. "You should have been the one to take my place. And one day you will. Death is not the end for you detective". The camera zooms in on Bruce's body underwater aging backwards into his younger form from earlier in the movie and his eyes opening as the screen fades to black.
Thoughs? Be it positive of negative, all is welcome