In the present, Minoru Akutsu, a 73-year-old inmate, finds himself nearing death in the solitude of his prison cell. Intriguingly, he is visited by an anthropomorphic balsam flower seated in a can, which taunts him about the perceived "pathetic" nature of his life. Despite this, Akutsu asserts that he still has the potential for a significant comeback. The flower reveals that only newborns and those on the brink of death can hear its voice. Their ensuing dialogue triggers Akutsu’s recollection of the events that led him to incarceration.
Flashback to 1987, where Akutsu is depicted as a low-level yakuza member residing in a humble seaside apartment arranged by his sworn brother, Tsutsumi. He shares this space with Nana Nagata and her infant son, Kensuke. Even though Akutsu is not Kensuke's biological father, the trio gradually forges a bond resembling that of a family. Despite his deep feelings for Nana and Kensuke, Akutsu struggles to articulate his emotions. When Nana proposes marriage, he declines, fearing that marrying her would put both her and Kensuke in danger due to his affiliation with the yakuza.
As Japan's economy flourishes, Akutsu's fortunes in real estate skyrocket, leading him to embrace a life filled with excess and indulgence. This newfound way of living results in his neglect of Nana and Kensuke, especially as Kensuke's health deteriorates. When Kensuke is diagnosed with dilated cardiomyopathy and an urgent need for a life-saving transplant in the United States arises, Akutsu is driven to secure a substantial amount of money.
In desperation, Akutsu turns to Tsutsumi for assistance. Tsutsumi proposes a daring plan to steal 300 million yen from their criminal organization's safe, which also requires eliminating Wakamatsu, a younger and increasingly powerful gang member. Akutsu, convinced that this heist is the only path to saving Kensuke, agrees. However, during the robbery, Wakamatsu is killed by Tsutsumi, leading to Akutsu's arrest as he takes the fall for the crime. While Kensuke's transplant is a success, Akutsu is sentenced to life in prison and is denied parole twice, primarily due to not being officially recognized as a family member of Nana and Kensuke.
Once confined, Akutsu finds himself deprived of contact with Nana and Kensuke, resulting in a heartbreaking separation. He has limited permission to send letters to Nana, which are scrutinized by prison staff that redact portions they find suspicious. However, Akutsu cleverly utilizes the censorship to his advantage. With his exceptional talent for visualizing and creating maps, he designs the letters so that the blacked-out sections serve as clues to the location of hidden money he had stashed away during the robbery.
Akutsu secretly retains about 70 million yen, a decision made to protect Nana and Kensuke’s future while preventing Tsutsumi from locating them. As the years roll by, Akutsu remains incarcerated, observing how Nana raises Kensuke, who survives his transplant and matures into adulthood.
Decades later, an elderly Nana returns to the apartment she once shared with Akutsu after being released from the hospital, where she too experienced the presence of the balsam flower. Utilizing the clues embedded in Akutsu's past letters, she pieces together a map that leads her and Kensuke to the hidden funds. In the process, Tsutsumi also catches wind of something left behind and searches the apartment frantically, but to no avail.
Along with the money, Nana stumbles upon a portrait of herself that Akutsu had drawn. Nestled within the artwork is a message where Akutsu finally expresses the emotions he struggled to reveal during their lifetime: “I love you.” This revelation leads Nana to understand that Akutsu’s vision of a "big comeback" was not about acquiring wealth but was instead aimed at securing their safety and freedom from the yakuza's grasp.
Meanwhile, in his prison cell, Akutsu gets wind from the balsam flower that Nana and Kensuke are safe, confirming that his long-thought-out plan has come to fruition. With a sense of peace, he reconciles with the reality of his impending death, passing away calmly in his cell, while the balsam flower remains faithfully by his side.
