Act I.
Interstellar Space.
Left off from the end-credit scent from Thor: Ragnorak. Loki and Thor battle Thanos' Black Order. They lose. Thanos compels Loki to give up the Tesseract. Loki is executed by one of the Black Order as Thor is forced to watch. Hulk escapes either via escape pod or Loki sends him back to Earth using the Tesseract. Heimdall perhaps gets away some survivors.
Space/Knowehere.
GotG warp into the debris field after Thanos has destroyed Thor's ship. They find Thor and he explains what happened. Thor convinces them Thanos must be stopped. The Guardians warp to Knowhere in order to retrieve the Reality stone from the Collector. Thanos beats them there, tortures the Collector and grabs the stone. The Guardians fight him but lose. They decide they have to seek help from Earth.
NYC.
Banner crash lands in Dr. Strange's living room. He tells Strange of the danger and Stark also shows up to discuss the issue. He warns Wanda to take Vision and hide. The Q ship arrives over NYC. Spiderman moves to intercept and is joined by Dr. Strange, Iron Man and Wong. They fight either the Chitauri or Outriders accompanied by members of the Black Order. I speculate they are after the Time Stone carried by Dr. Strange or trying to lure Vision out. The invasion is halted, but the Q ship escapes, damaged.
Act II.
Earth, major city. Night.
Vision, Wanda, Black Widow, and Cap fight the Black Order who have come for Vision. The Order are defeated and flee.
NYC.
Stark and the others talk about Thanos and form a plan. I think at this point the Guardians show up. The basics are Cap's team with Banner will take Vision to Wakanda using their superior technology to 1) Fight Thanos, and 2) to find a way to keep Vision alive if the Mind Stone is taken from him. Stark's team will travel to Titan with some of the Guardians to retrieve the Soul Stone that is said to be there. Thor and Rocket and Groot will travel to some destination in space to forge a new hammer/axe for Thor. But there is a secret plan in the case of failure.
Wakanda.
Cap and Co. arrive in Wakanda, seeking T'Challa's aid in saving Vision and fighting Thanos. He agrees and Shuri and Banner begin researching how the Mind Stone works and how Vision can live without it. Wakanda is put on war footing.
Titan.
Stark, Dr. Strange, Spiderman, Star-Lord, Drax, Gamorra and Nebula arrive on Titan. Titan is a world long since devastated. The Q Ship crashes nearby. Thanos is already there and already has the Soul Stone. They battle. Thanos kills Spiderman and Gamorra, and captures Dr. Strange. Stark and Star-Lord are forced to flee.
Sanctuary.
Dr. Strange has deactivated the Time Stone. Ebony Maw tortures him and eventually uses his powers to persuade Strange to activate the stone. Thanos adds it to his gauntlet which now features Time, Power, Space, Reality and Soul. He sets off to retrieve the Mind Stone.
Act III.
US City. Possibly San Francisco.
Scott Lang is going about his business when he runs into Stark and Star-Lord. Stark asks him how small he can shrink and about countless other sub-atomic universes...
Wakanda.
The Outriders begin their invasion with drop-ships. T'Challa, the Royal Guard, Bucky, Cap, Black Widow, Banner in the Hulk Buster armor and the rest, minus Wanda and Vision, engage the Outriders. The battle rages and Thanos appears. He promptly makes short work of the Wakandans. He kills Captain America and finds Vision and grabs the Mind Stone. Thor arrives with his new hammer and engages Thanos. He probably kills other heroes.
At the end, Thanos adds the Mind Stone to his gauntlet. He raises his hand and simply snaps his fingers as the gems glow bright all at once.
Cut to Black.
End Theory I: Vision was relieved of the Mind Gem at some point while in Wakanda. Thanos doesn't have the real Mind Gem.
Mid-Credit Scene: Clint and Laura Barton are get a package in the mail. They open it. It is the Mind Stone.
End Theory II: Stark and Lang use the Ant-Man suit to shrink through time. They either intend to replace the Mind Stone or Time Stone at some point in the past, or travel to a different dimension of the multi-verse to replace the Mind Stone in Loki's scepter. In the comic books, the stones will only work together if they are all from the same dimension in the multi-verse.
Mid-Credit Scene: Tony Stark and Scott Lang materialize in 1997. Tony says he hates the 90s.
