The setting is August 2018. The first character we encounter is a former confidence man with one past criminal conviction, he is currently a bachelor with no girlfriend and no friends, and he calls himself Rotten Basement Banana (hereinafter “Banana”). Inspired by a self-help book titled Your Friend is You Yourself, he decides to try to find a friend online by writing a self-introduction on a matching app.
After getting one response from a woman who calls herself Mika, he receives 131 more messages, but they are all clearly phishing scams intent on fraud. Realizing all 131 messages are written by the same con artist, Banana dubs her “Hyakusaisakura” (100-year-old Sakura) (hereinafter “Sakura”) and sends her emails reprimanding her and telling her to mend her ways.
At first Sakura denies the accusations, but eventually Sakura proposes that they meet anyway, while admitting that he is actually a man, not a woman. When they do meet they find out that they have a lot of things in common, and finally they decide to start a private detective business together.
On the beat in their new profession, Banana and Sakura a man named Mitsuo who is supposedly the boss of a number of illegal back-street businesses. In order to get information about this Mitsuo character, they contact his former girlfriend, a call girl named Rena. When she finds out what their purpose is, she resists at first, but in time she ends up in an affair with Sakura.
The summer of 2019. From Banana who has been caught and being held by the man known as Mitsuo, Sakura gets a video phone call. The uses the letter-reading function of the smartphone to say that he is probably this Mitsuo character they are looking for, but he has lost all memory of his former self. He says that he has the ability to predict when a specific person is going to die, and he says that Banana is going to die on September 30th of 2020. Because of the scar on his neck (kubi in Japanese), Banana has given him the name Kubi-chan, Banana has asked to lock him up and protect him just on that day he is supposed to die. The man has agreed, and through this exchange he becomes friends with Banana and Sakura and joins up with them.
A woman named Arisa is lying on a bed, her body writhing, seemingly in pain. She says she wants to get drunk and lie in peace until the morning from which she will never awake again.
The year is 2017. Banana meets Arisa at a teetotaller society for people trying to stay off drinking. Arisa is under the delusion that Banana is her former upperclassman from their junior high school brass band club, and Banana finds himself unable to deny that connection. After the teetotaler meeting, he asks her out to tea, having fallen in love with her…
Banana is remembering this encounter two years later in the winter of 2019. Banana says he wants to make it a new mission of their private detective agency to search out Arisa and save her. Ever since Rena’s former boyfriend Kubi-chan (Mitsuo) has started working with the two private detectives, Sakura feels that his relationship with Banana has gone sour. Banana says that both Sakura and Kubi-chan are important to him, and the three of them begin working to save Arisa.
The year is 2017. Rena has become a cocaine dealer for Mitsuo.
The year is 2019. From their research, Banana and his partners have found that Arisa’s is living in Fukushima Prefecture under the name of Kanae. In order to get close to Arisa, who appears to carry a deep distrust of men, Banana asks Rena to befriend Arisa. Part of the reason for this request is for Rena to resolve the rift between her and her former boyfriend Mitsuo (Kubi-chan) who is now wrong on the detective mission as well. Rena accepts the proposal.
Arisa is placing flowers and a bottle of [alcoholic] drink at what appears to be a grave. After putting her hands together in prayer, she rises to leave, as if being drawn by something, returns and reaches for the bottle. A ghost-like man appears and starts to beat her. She fights back and finally kills the man before collapsing herself in fatigue.
The scene suddenly becomes Kanae’s (Arisa’s) room. It is February 2020. Rena, who has now become Arisa’s friend, arrives for a visit. Arisa wakes up a man who appears to be her client as a prostitute and drives him out. Rena says that she is leaving today to return to Tokyo, and she invites Arisa to come with to hear some musician friends of hers perform. Little does she know that the outcome will be tragic. Looking back as she recalls what happened, Rena is full of remorse.
At a live [music] performance club. Wearing masks, Banana, Sakura and Kubi-chan are performing as a band. Listening to them are Rena and Kanae (Arisa). As Banana is singing the song, I Love You, he takes off his mask. Arisa is surprised to see him. Rena finally reveals to Arisa the whole story of who they are and how they came here, but Arisa won’t accept it as true, and when Banana confesses his love for her, she says, “Let’s just be friends.” Rena introduces Arisa to Sakura, and Sakura introduces here to Kubi-chan. When Arisa sees Kubi-chan’s face, she is suddenly disturbed. Believing that Banana and the others have conspired to bring Mitsuo to her, Arisa runs away.
Rena says that when found on the streets in a deranged state, Arisa was taken into custody and put in an institution. She goes on to say that Banana has died, Mitsuo has disappeared and she has broken up with Sakura.
It is September 30th of 2020. This is the day Banana was told he would die. He is tied to a chair in their office. Ever since that encounter with Arisa and her disappearance, it seems that Banana has started drinking even on the job. Sakura says that after being untied from the chair on the pretense of taking a commemorative photo, Banana began playing hide-and-seek, escaped from the office, and after involving the entire town in a game of hide-and-seek, he ended up dead.
Kubi-chan says that he has regained his memory of being Mitsuo, and he confesses that he intended to tell everyone after he had saved Banana. He goes on to predict that Arisa will die on March 24th of 2021. After getting into an argument about the deaths of the two, Rena and Mitsuo leave Sakura alone.
The date is January 1st of 2021. Sakura receives a message from Banana. It is a message that Banana had recorded for when he died and made a reservation for it to be sent on this day. In it, he writes that he wants Sakura and Rena to name their child “Hana” (Flower), and when they have time, he wants them to try to clear up Arisa’s misunderstanding of all that happened.
The date is March 24th of 2021. With the intent to save Arisa from her predicted death, Sakura appears where she and Rena are. When Sakura asks Arisa if she wants to live, she replies, “Of course, now I want to live.” At some place beyond the sight of the three, Mitsuo is holding down the God of Death. The God of Death escapes from Mitsuo’s grasp, and when a man that looks just like Banana in its place, he holds him down.