What was it that killed him?

Ayato Kazama is repaying his deceased father's debts and caring for his mother, who suffers from a serious illness, while working at a construction site during the day and at the karaoke bar his parents opened at night. His brother, Sohei, lives with him and also takes on the debt repayment and caregiving, dedicating himself daily to becoming a mixed martial arts (MMA) fighter, following in their father's footsteps. Despite their suffocating life, Ayato longs to seize a small piece of happiness with his girlfriend. However, on a night meant to celebrate his best friend's wedding with a modest yet happy gathering, their simple daily life is shattered by unexpected violence.