The Avengers will go to fight Thanos near the beginning of the movie but will be easily defeated.
Thanos uses the soul stone to drain Captain Marvel’s powers but Thor saves her before Thanos kills her.
Thor then uses Stormbreaker to transport everyone back to Earth where Banner is treating Tony Stark (who made it back to Earth with Nebula).
When Banner and Stark are analyzing Captain Marvel and her weakened state they are stunned to know that she’s powered by the Tesseract (Space Stone). They then realize that they can replicate Infinity Stones. Thor and Captain Marvel go off to find the remaining Asgardians, while Banner and Rocket try to figure out a way to time travel with the help of Antman who’s out if the Quantum Realm. They do.
The Avengers Reassemble and use the Quantum Realm to go back in time to each place they remember seeing Infinity Stones.
Nebula tells them that the Soul Stone is on Vormir and that in order to get it that they need to sacrifice someone they love. Captain America rejects that idea saying “we’ve lost too much already”.
Banner, Captain America, Iron Man and Thor make a plan to fight “past Thanos” on Vormir.
Thor almost defeats Thanos “again” before he can use the Power Stone. Iron Man is able to stun Thanos with the new Mark 85 so Banner can copy the Power Stone and they retreat back to the present via the Quantum Realm.
“Present Thanos” now knows their plan and is waiting for them at Avengers Headquarters with the Outriders by the time Thor, Banner, Captain America and Iron Man exit the Quantum Realm.
Thanos attacks at Avengers HQ and Hulk finally comes out for his rematch against Thanos. Thanos uses the mind stone on Hulk to turn him against the Avengers but ends up merging Banner and Hulk, enter…Professor Hulk!
With knowledge that they can replicate Infinity Stones, Iron Man has begun working on the “Stark Gauntlet”. During the attack on Avengers HQ, Thor and Captain Marvel are tasked with defending the Stark Gauntlet.
Thanos nearly defeats Hulk (Professor Hulk) again before Thor comes and saves him. The rest of the Avengers go on the attack before Thanos uses all six Infinity Stones to destroy Avengers HQ and stomps on their “A” logo. Thanos then begins to give an epic speech on why he did the finger snap then is abruptly interrupted with a HUGE blast by War Machine wielding the Proton Canon! A wounded Thanos is able to retreat using the Space Stone.
With Avengers HQ and the Stark Gauntlet destroyed, Ronin tells Tony of another way they can forge a Gauntlet (this is the reason Hawkeye was in Japan, now known as Ronin).
Iron Man is able to forge a new Gauntlet and Professor Hulk uses it (only Hulk and Thor are able to use the new Gauntlet) to bring back all the dusted, but in doing so the Stark Gauntlet explodes and Professor Hulk loses his arm in the process.
Now that all the dusted are back, Iron Man builds Professor Hulk a new arm and tells EVERYONE that they must fight Thanos. Whatever the cost, “whatever it takes”.
Doctor Strange then teleports everyone to Thanos and everyone fights Thanos together at once. One of the most epic scenes in movies, EVERYONE (superheros) who got dusted are now all together fighting Thanos.
Thanos is able to hold them off before Thor swoopes in with Stormbreaker and his able to cut off Thanos’ left arm with the Inifinity Guantlet. Captain America then races over to the Guantlet before Thanos can grab it and uses it to teleport Thanos away before he is killed due to wielding the Infinity Gauntlet.
Only Thor and Hulk could use the Inifinty Gauntlet without dying, though Hulk lost his arm. Captain America knew this but made the sacrifice the teleport Thanos away. Note: Thanos was not killed, similar to Red Skull in Captain America: The First Avenger.
Everyone is at Captain America’s funeral. Iron Man gives Captian America’s shield to Bucky Barnes, and now he takes over as the “New” Captain America. Ronin leaves with his family, leaving his bow and weapons behind. Banner takes off to learn more about “Professor Hulk”. Natasha takes off saying she has “unfinished” business (leading into her solo movie). Thor leaves to be king of the “new Asgard” in Norway.
Post Credit Scene:
As the last people are leaving Captain America’s funeral. The camera zooms “ground-level” to show someone coming in a wheelchair. As the camera pans up to waist-level the screen goes dark and you hear “good evening Mr. Stark, my name is Charles Xavier”