system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The plot to Bioshock Infinite

I loved this game, and think I have a good grasp on it, but I would love to hear an ELI5 version!
gpt
Let's do this chronologically.

There was a guy named Booker Dewitt. He fought at the Battle of Wounded Knee, killed a lot of indians, and felt really bad about it. After the battle, he found people being baptized in the river. At first he volunteered for the baptism.

Here, the universe splits. It becomes World A, in which Booker does not go through with the Baptism, and goes back to his life and becomes a strike buster. In World B, he becomes Zachary Comstock, remains in the military and goes to Columbia. 

Booker in World A eventually has a baby, Anna. Comstock, in world B, is unable to have a baby because of experiments done by Leteuce. Comstock in world B wants to have a baby, and Leteuce thinks that she can get him one from the world of her "brother" (who is really her in the alternate universe, similar to how Comstock/Booker are the same person). Booker has a ton of gambling debts but no way to pay off the debts, so Male Leteuce says he can pay off the debts by giving them his baby. He agrees to do that, and they take the baby, but Booker changes his mind and chases them. Leteuece gets Anna through the rift, but part of her pinky gets cut off and stays in Booker's world, meaning she exists in both World A and World B at the same time. This gives her the ability to open portals between worlds. These rifts can also span across various times (for example, in one early scene Elizabeth opens a rift to NYC, and the theater marquis shows "Star Wars" being played, even though it's only 1912 in the game).

Booker sinks into alcoholism and depression after losing his baby, carves her initials (AD) into his hand, and gets into more debt. Leteuce again offers to wipe away the debts, if he goes to this city and brings a certain girl to New York. He agrees, and they bring him out to Maine, and pass through a rift at some point without him knowing. He goes into the light house and goes up to Columbia, rescues Elizabeth from her tower, they flee the city, etc. Elizabeth is captured by Songbird and brought back to Comstock's house. Future version of Elizabeth brings Booker to the future and shows him what happened as a result of not having stopped Songbird from taking her, then sends him back to stop Songbird. Elizabeth opens a tear to Rapture (from Bioshock 1) and Songbird is killed by the water pressure. Then they go up to the lighthouse. Each lighthouse represents a "World" created by a choice (like, Booker's choice to get baptized or not). Each world branches off from many different important choices. Anna realizes that as long as there is a world in which Comstock exists, there will be a world in which she ends up destroying everyone using Columbia. So, she opens a tear and they go back to the moment where Comstock began: the baptism. There, the many versions of Elizabeth that had all reached the same conclusion meet up and drown Booker before he can become Comstock, thus preventing that horrible future from ever happening, and that's why they all disappear.