willofD.evilfruit

3. BSD Tale of the Lost is set before the events of the anime (and only mentioned a couple of times in it) featuring Fyodor D. and Tatsuhiko Shibusawa. Better known as the 88 days lasting Dragon Head Conflict. The events that happened here are not shown in the anime, but are hugely related to the movie. Dazai was 16 here.

4. At age 16 Dazai also becomes the youngest Port Mafia executive.

5. From 15 to 18 Chuuya and Dazai are sworn rivals and after they decimate an entire enemy organisation in one night, at age 16, they become known as ‘the criminal world’s worst enemy’ earning the title Twin Dark/Double Black, (depending on the translation) ending up as Port Mafia partners.

6. When Dazai is around 17 he finds Akutagawa and makes him his apprentice in the Port Mafia

7. When Dazai is 18 is when the Port Mafia fights Mimic. Oda’s story happens, telling Dazai that if it’s all the same to him, he should join the good guys. Dazai learns that Oda dying was part of Mori’s play to obtain the license and takes Oda’s last words to heart. Dazai leaves mid-mission, abandoning the Port Mafia and asks Taneda for job advice. He tells him about the Armed Detective Agency, but also says that with his record, Dazai needs to go into hiding before he can start the job.

8. Dazai stated (in the manga) that in his 2 years of hiding he was an unemployed, unmotivated drunk in bars, but we don’t know for sure if this statement is true.

9. At age 20, after he’d been in hiding for 2 years, Dazai joins the Armed Detective Agency listening to Taneda’s recommendation. Fukuzawa puts him in Kunikida’s care. Kunikida decides that a serial disappearance case becomes his entrance exam. After passing it, Kunikida and Dazai become partners at the ADA.

10. Another 2 years later, at age 22, is when the anime starts. Dazai meets Atsushi and lets him join the Agency

11. The main plot events from season 1 and 2 happen, until after the Guild is defeated. 

12. Next is the Dead Apple movie. The movie begins with a scene that is said to be 6 years in the past. 22 - 6 puts us at when Dazai was 16: the time of the Dragon Head Conflict. It then cuts to the present, but it’s made clear that the movie is connected to events from the Tale of the Lost. In the movie, Ango calls Chuuya for help. Chuuya makes clear that, in the past, Ango betrayed them by bringing Tatsuhiko Shibusawa to Yokohoma, which lead to the death of his friends. An event that we didn’t see, but most likely happened in Tale of the Lost. This is why Chuuya says that he wants revenge and to kill Ango as a reward for helping him. The movie is related to a lot of past events that involved Fyodor and Shibusawa and shows that Dazai and Chuuya already have a history with them. I assume that serves as a bit of an explanation for their crazy trust relations in the movie. 

13. Then season 3’s main plot events happen.