The boys meet when they are punished for being late to school. They are tasked to clean a storage room in their school. This became their gathering place, and where they created many memories.
Their brothership was important to them. However, each of their own struggles pulled them apart, and Namjoon moved away. The principal forced Seokjin to tell him where their hideout was, which lead to Yoongi and Jungkook being walked in on while playing the piano. The teacher that had found them slapped Jungkook, and Yoongi retaliated. This got him expelled, and broke them apart more, as Jungkook blamed himself.
Jimin and Hoseok were at a bus stop when Jimin had a seizure. Hoseok brought him to the hospital, but was not allowed to stay because of his mother. A few weeks later, Jimin lies for the first time in the hospital. He had a traumatic experience at the Flowering Aboretum, and since then he's been having seizures.
Jungkook kept hope of seeing all the boys together again, and visited the hideout daily. They never ended up meeting there again, only Jungkook and Hoseok. His family was the boys, but they fell apart. He was closest to Yoongi.
Namjoon had moved back after a year,not telling anyone, and ended up living in a shipping container. Seokjin moved to a school in LA, also without telling anyone. They never knew where Taehyung lived. Yoongi never came back to school after being expelled. Jimin stayed in the hospital, also never going back to the school. Hoseok worked a part time job. Jungkook, presumably, was still in school.
Seokjin came back after almost 3 years, on April 11. He went to a gas station and found Namjoon working there. He did not, however, say hello. A month later, he decides to see where the boys are now, and goes to see Namjoon. When there, he finds out that Namjoon is in jail. He goes to visit, where he learns many things. Hoseok was in the hospital after an accident. Namjoon didn't know where Jimin and Taehyung were, but knew that Jungkook and Yoongi were dead. While leaving, he saw Taehyung tied up outside, being convicted of the murder of his father.
Seokjin decided to go back to the beach where they visited, while wondering what went wrong. A voice asked him that if he went back in time, did he think he could save them all? He woke up, and it was April 11 again.
Seokjin didn't believe it, and went on with his day. He went to the gas station, saw Namjoon, didn't say hi, and went to drive through an alley. As he drove through, a body hit the hood of his car. He realized it was Jungkook. Everything around him started shuddering, and it all cracked. It was April 11 again.
He still though it was a dream, and so kept going. He went to the gas station, didn't talk to Namjoon, and walked down the alleyway. This time, Jungkook's body dropped behind him. The world shattered one again, and the voice spoke to him. "You won't make it out here alone in this tangled destiny. Here's how to remember the loop. Once you open your eyes..." Seokjin now saw the different timelines. He realized he's been going back in time. He know was able to figure out how each member died, though going back in time had some changes, as caused by the butterfly effect. We know how Yoongi and Jungkook died, shown in the "I NEED U" and "Euphoria" music videos. We see how Taehyung kills his father in the "I NEED U" mv.
Seokjin goes back numerous times, but was not able to save all of them. He tried to keep notes of who was where, what was happening, but couldn't save them all.
There is an order of the events that occur to he boys. On April 11, Namjoon gets into a fight with a customer, and goes to jail. Later on April 11, Jungkook falls off a building and dies. He wasn't in contact with any of the boys, and his family didn't seem to care. On May 2, Yoongi burns himself in a hotel or motel. He also tries to burn himself of April 29 and May 10, changing because of the butterfly effect. Yoongi kept himself distanced, and was always drunk, thinking he would hurt people. On May 10, Hoseok falls and hits his head on a bridge, going into a hospital for narcolepsy. May 12 Jimin tries to drown himself in a bathtub at the hospital. He has been in the hospital the whole time, as his parents were ashamed of his sickness. May 20 Taehyung kills his father. He saw his father hitting his sister, and snapped. He broke an alchohol bottle and used it to stab his father. In another timeline, on May 22, Taehyung jumps off a viewing platform by the sea, in front of the boys.
Seokjin finds this all out after trying to save them all many times. He did, however find solutions. Approaching Namjoon at the station before he can get in a fight. Rushing over to the construcvtion site to save Jungkook. He has to figure out which hotel Yoongi sets a fire in, and go to save him. He saves Hoseok from falling, and then breaks Jimin out from the hospital. They all gathered together, and everyone was happy. But he had forgotten about Taehyung killing his father. He gives Taehyung a ride, hoping to get Taehyung to tell him something. When he heard screams from inside Tae's house, he realized he needed to do more.
He realized he couldn't save them by himself. Theories say that he went through many more struggles trying to save all of them. Taehyung has dreams of the past timelines, and it is thought he might see the future ones.
It is theorized that Seokjin left clues and notes for each of the boys to see, so they could save each other. Seokjin meets up with Namjoon, saving him from the fight. Hoseok ends up inviting them all to a party in Namjoons container house. Yoongi calls Jungkook to save him from jumping. Jungkook saves Yoongi from the fire he sets. Hoseok still falls, and at the hospital, Jimin saves him from falling down the stairs, which means that Hoseok got discharged. When he met the boys again, they went to break Jimin out. Jin uses Hoseok to save Taehyung. Hoseok drops him off, and Seokjin asks him to call Tae. This stops the fight, and Tae doesn't kill his father.
Seokjin was able to get past May 22, the day he was stuck on, by keeping them alive. They all went back to the ocean. Taehyung had suspected that something was wrong, and when Seokjin went to tell them something from high school. Taehyung asked if he was going to tell them about him betraying their spot and getting Yoongi expelled. They broke out in a fight. Seokjin goes back again, and this time Seokjin leaves before the fight. He climbs the tower, but goes back down after Tae climbs it. He leaves, and drives off leaving the boys with no ride. Later that night, Jungkook gets hit by a car, which is suspected to be Seokjin. They fell apart after May 22.
Hoseok learns about Jungkooks car accident first. He goes to the hospital with Jimin when Jungkook comes out of his unconsciousness. Taehyung comes, but couldn't get himself to go in. Jungkook meets a girl that helps him get through his recovery. Yoongi meets a girl that helps him make music again. He became consumed in work, and his phone wasn't used. When he charged it, he got a call from Jimin telling him about Jk's accident.
Yoongi couldn't get himself to go into the hospital. However, at the hospital, the girl Yoongi worked with was palying a song that Yoongi played in HS, and Jungkook recognised it. Taehyung is alone, but sees a girl stealing from a store. They meet a couple times more, and become friends. They put graffiti on a bus stop, and when running from the police, Taehyung gives himself up. After escaping the hospital, Jimin lived with Hoseok. He went to a dance class lead by Hoseok, and collided with Hoseok's friend. He immediately runs to the bathroom, where he cleaned himself off, as the blood reminded him of the Flower Aboretum. Hoseok runs to the hospital with the girl, and hurts his ankle. He discovers a plane ticket to the US in her backpack. Namjoon becomes interested in a girl he sees daily, leaving her a hairband.
Eventually, the boys find themselves, wanting to become better people. Taehyung wants to protect his family. Hoseok was healing his ankle, and toured around with a dance group. He decided, at the end, to go back home. Yoongi lost contact with the girl, but continued making music. Jimin kept practicing dance, wanting to commit to something. Namjoon wanted to go back to his HS passions. Jungkook was slightly off after the car accident. He was trying to figure out what happened that night. Hoseok wanted them to group up and see the fireworks in August together. After this, they were talking more again.
In July, Jin saw a girl drop her diary. It had her wishlist in it. They meet up, and fulfill her wishlist. One thing she couldn't get were Smeraldo flowers. He found a shop to get them, but they wouldn't come until August 30. He wanted to confess with the flowers, under the fireworks on August 30. Unfortunately, he couldn't. Seokjin was able to get the flowers, but there wasn't a card. He called the company, and they turned around the truck. At the same time the girl was crossing the road, and the truck hit her. Seokjin just stood there, but heard a crack. He likely went back in time, to the morning before he met the girl. There was a clip where a vase fell, and it occured again, but Seokjin caught it, leading to believe he travelled back. He ends up not givng her flowers, but her diary back. He realizes that by doing everything in the notebook, he wasn't himself.
On August 3, they went back to their school hideout. Seokjin seemed to not know about their memories together. It is suspected Jin lost his memory. Seokjin tries to leave, but they get into a physical fight. This breaks down a wall they didn't know was there, revealing a chained cabinet. They open it up, and find Seokjin's father's diary. It had a note on a date, saying that this is where it all started. Many think that Seokjin's father also could time travel.
This is the end of the definitive story. Many other theories arise about what happens, and there are more online resources.