Yasuo is a gangster just released from jail. Believing his boss double-crossed him, enraged Yasuo is on his way to find the boss. But before setting out for the dangerous trip, Yasuo asks Kenji to take care of his mentally challenged sister, Yuri, and a mysterious black bag.

Kenji, abandoned by his mother, scrapes out a meager existence doing odd jobs including driving bar hostesses and their customers home. Besides this he takes care of the sister of an old friend in jail and a young illegal immigrant. Eventually his life reaches a turning point.

Eureka tells the story of an orphan who survives a traumatic experience when a bus he is on gets hijacked in rural Japan. The incident leaves him with survivor's guilt and post-traumatic stress disorder (PTSD). The movie explores themes of trauma, starting over, and the loss of loved ones. As the orphan tries to cope with his past, he forms a close bond with his cousin, who also experienced the hijacking. Together, they navigate through their shared traumatic experience and find ways to move on.