This goes without saying, but *spoiler alert!!*
Slaughterhouse-Five deals with a man named Billy Pilgrim, born on the 4th of July, 1922, who has become unstuck in time. This means that he 'uncontrollably' travels to his past and to his future and lives out the events that occur until he travels again.
"poo-tee-weet"
As a young man, Billy attended the Ilium School of Optometry until being drafted into the Army at 21. Billy was very unprepared for war and was soon taken prisoner in the battle of the bulge with the man who saved his life, Roland Weary. Billy was doing things overseas that would have killed any man and Roland Weary protected him like an older brother. When they were captured, Weary blamed Billy and eventually died of gangrene.
so it goes
Weary's dying wish was for a man, Paul Lazarro, to avenge his death, meaning that he wanted Billy Pilgrim dead. But before Paul could carry out this wish, they were transported from Luxembourg to Dresden. They were forced to settle in a slaughterhouse called "Schlachthof Funf" or "Slaughterhouse Five". The allied forces ended up bombing Dresden later on and completely leveled the city. Billy and some other POWs survived in a meat locker and were forced to clean up the bodies.
so it goes
When the POWs are liberated and transported back to the US, Billy is sent to a psychiatric hospital since he had difficulty coping with what he experienced in the war. His girlfriend, Valencia Werble, whose father owned the Ilium School of Optometry, accepted his offer of marriage and they had a son named Robert and a daughter named Barbara.
so it goes
Based on the story's perception of time, it was his daughter's wedding night that he was abducted by aliens called Tralfamadorians who could see in 4 dimensions and taught him that there is no such thing as free will; every decision has already been made and we are only here to figure out why they were made. Billy learned a lot from these creatures after being held prisoner and was sent back to Earth unharmed.
so it goes
Soon after his return home, Billy was in a plane crash and was the only person to survive. His wife died of carbon monoxide poisoning on her way to visit Billy in the hospital. When trying to tell his daughter of the Tralfams, she told him he was senile and must have hurt his head in the crash.
so it goes
In the future, something he relived many times when he was unstuck, Billy gave a speech about what he experienced with the Tralfamadorians and his perception of time. It is after this speech that Paul Lazarro murders him with a laser gun.
"poo-tee-weet"
Hope you aren't as confused as I was.