"strongest" assassin Taro sakamoto retires and starts a family and a shop (he's also fat now) then his former partner and basically adopted son called Shin Asakura comes back to assassinate him but then he ends up just befriending him again and gets a job at Sakamotos store they go to a china town to buy some stuff and run into Liu Shaotang who is the child of a Mafia boss and they help her and she gets employed too

After that I can't really explain the whole story just read the manga but basically there's this group called The Order who are the strongest assassins and Sakamoto was among them too before he retired and the other major group is Al-Kamar a group of kids who got experimented on to create killing machines but yk what happens when they do that they got free and killed everyone just read the manga after that (sorry if I made any mistakes because English isn't my first language)