First things first: Last night's leaks were right, sadly. Tony and Nat die for good (Tony when snapping Thanos' army, Nat is Gamora 2.0). Cap goes back in time in the end and comes back an old man.
Indeed all promotion material is nearly only the first 20 minutes.
First scene is Hawkeye losing his family to the snap. Last scene is Cap and Peggy dancing in their own little house in the past.
No post credit scene, only mid credits "Twilight"-style. The original six actors signed their autograph between their first and last names.
After Hawkeye scene, Marvel card. Then Tony and Nebula playing some game with metal scraps. Then Tony recording his message, including flashbacks to the last 22 days with Nebula. She heals his stab wound there.
Captain Marvel then shows up. Post credit scene from Captain Marvel is not shown again, that happened before this rescue. CM brings the ship back to Earth. Everyone from the CM post credit scene plus Pepper waiting there. Tony is in a bad shape.
Tony and Cap fight, Tony blames Cap still for the Civil War events.
They fly to fight Thanos (CM post credit scene Avengers, not Tony). Thanos has destroyed the stones, that was the thing he did "2 days ago". They beat him and Thor cuts his head off. They know they lost.
5 years later time jump. In that time: Bruce learns how to become Dr. Hulk. Loves it, best of both worlds for him. Tony and Pepper marry and have a kid. She's fiveish when events pick up again. Thor establishes a fisher town for the remaining Asgardians including Valkyrie and becomes a drunk with a dad beer pod. Rhodey searches for Hawkeye who goes on a killing spree for every villain out there around the world. Steve has a support group. Nat holds the fort in the Avengers facility. They tell each other that they have to move on but can't. Okoye runs things in Wakanda.
Scott's van is dusting in some storage facility. He comes back when a rat runs over the control that Hope couldn't press anymore before she was dusted. For him only 5 hours passed. He reunites with Cassie and goes to the facility.
Scott figures they can fix things with time travel. No alternate dimensions come into play though they're mentioned. They start recruiting the rest of the team, including Thor in his fisher town (Rocket and Banner go there). Tony doesn't want to help first because he doesn't want to lose what he built in the last 5 years.
Bruce works on the time travel alone then. In the facility, Soctt is testing the suit. He gets turned into an old man, a baby and a kid. It doesn't work.
Then Tony arrives and agrees to help, but only if the last 5 years remain untouched and they are, they are never reversed. He brings the bracelets which stabilize the time traveling.
Nat finds Clint in Asia and tells him, they have a chance. He asks her not to give him hope. She says she's sorry for not being able to do it before. Clint is the first to test the suit, he goes to his farm, wants to go see his family but is yanked back before he can.
They go after the stones. They all have only one try because Scott has only so little Pym particles left.
Nebula flies with the Guardians ship, Rhodey, Nat and Clint. Nat and Clint drop them off to get the power stone and go for the soul stone with the ship. Nat does Gamora 2.0 after Clint and her fought over who gets to die (she rips herself away from his grasp and falls, dies and is never seen again).
In 2014, Nebula and Gamora are still loyal to Thanos. In a conversation between them, Nebula's neural interfaced turns on by itself and shows a memory of current events because it is connected with her interface in the future. Thanos then takes her prisoner because he doesn't trust her and her interface is tapped by Thanos. He sees everything going on in the future including his death. He, old Gamora and old Nebula travel to the power stone planet and take Nebula prisoner. Old Nebula switches her golden face plate with new Nebula's to impersonate her and go back to 2019 with the power stone.
Thor and Rocket go to Asgard. Rocket gets the Aether out of Jane with some device. Thor has a long tolk with his mother. Before they leave, he gets his hammer.
Scott, Cap, Hulk and Tony go to 2012 New York. Hulk retrieves the time (EDITED, WAS A TYPO) stone from the Ancient One. She gives it to him after Bruce tells her that Strange has given it away. The Ancient Once says that Strange is the best Sorcerer (which she knows because she can see the future), and that she counts on Hulk.
Tony and Steve try to retrieve the scepter and Tesseract from the Avengers tower at the point of time, right after the New York fight. Old-2012-Loki snags the Tesseract, leaves with it and is never seen again. Steve gets the scepter but runs into his own old self and has to fight himself. He beats himself with the scepter. Cameos by Pearce and Rumlow. They reenact the elevator scene from Winter Soldier in the Stark Tower, basically, but no fight. Steve says "Hail Hydra" to convince Crossbones to give him the Tesseract.
Then Scott takes the scepter back to 2019.
Tony and Steve go back to 1970 because in the old SHIELD facility there there's not only another Tesseract (from that year timeline) but also some Pym particles there. They learn to trust each other again in this course. Tony has a talk with his father and reconciles with him. They snatch the Tesseract from that year and the particles and go back to 2019.
Hulk puts on the gauntlet with all the stones. Thor wants to but they convince him Hulk is the better choice. Hulk loses his arm (EDIT TO CLARIFY: It's still attached but burned so bad he can't use it. In the end he wears it in a sling), but does the snap. Nothing happens at first.
In the meantime old-Nebula has opened the time tunnel for Thanos' ship with his army and him from 2014. The others don't know because Tony sealed off the room for the snap. Thanos comes through the portal and old-Nebula destroys it. Thanos then destroys the Avengers facility with one blast. This is where the final battle starts and takes place.
New-Nebula convinces Gamora to free her and they fight - and kill - old-Nebula.
Rhodey and Rocket are nearly killed by underground floods but Scott saves them. Clint finds the gauntlet underground and carries it around (not putting it on), always on the run from Thanos' army.
Thor, Tony and Steve fight Thanos. Thor has his armor back on, his beard braided and looks much better. Cap wields the hammer.
Thanos nearly wins. Then the dusted come back through a lot of Strange's portals from wherever they were dusted. They get NOT resurrected with those portals, they just appear where they died and Strange and Co. bring them to the Avengers facility through the portals. T'Challa, Shuri and Okoye bring the Wakanda army with them. Peter reveals that they all don't know what happened, except for strange. For the dusted, no time has passed.
Steve says "Avengers assemble". Epic fight ensues. This is the big one, this is where my source cried because it's so epic. Tony and Peter take a moment for a reunion. Pepper is there as Rescue, no back story there. The goal here is to get the gauntlet to Scott's van which is somewhere on the battlefield so they can bring the stones back and undo Thanos' new attack. Includes an awesome scene where basically all female fighters on the field carry the gauntlet part of the way. But they all have a hand in carrying it there.
They fail tho, Thanos puts the gauntlet on. Tony stops him from using it after Strange showed him with one finger up-motion that this is the only way, and puts it on himself, snapping Thanos' army and himself. Thanos dies sitting sadly on the ground.
Tony dies after Peter, Pepper and Rhodey said good-bye. Later Pepper plays some BARF video showing Tony saying good-bye (he recorded it before they go for the stones). Then they say good-bye to him in a funeral by the lake. General Ross and Hank Pym cameos. No Betty in the whole movie.
Steve takes it on him to bring the stones back to where they took it fromm using another quantum tunnel. Bucky apparently knows he's not coming back, they say good-bye for "long" while Sam and Bruce think he'll be back in seconds. But he doesn't but shows up an old man instead a moment later, sitting on a bench. He had his life with Peggy (he shows his wedding ring but doesn't want to talk about it). He gives the shield to Sam. Bucky approves.
Peter goes back to high schol and meets Ned, who is very moved. EDIT: Source says it's a school but she's only seen Homecoming once and is not sure it's THEIR school, maybe College?
Thor gives the throne of Asgard to Valkyrie and leaves with the Guardians, bickering with Peter about who's the boss. He has no idea what he wants to do next.
Clint goes to live with his revived family.
Last scene with Cap and Peggy dancing. The end.
Misc answers from questions:
No vision. Wanda is very angry fighting Thanos, says, she lost everything to him. He says, he doesn't even know who she is.
Literally no idea who Katherine Langford is playing since source doesn't know the actress and source says, end credits race by crazy fast.
