She waited 400 years to become his wife.

Shunsuke Takeda works for a local government office in Tokyo, and without any love interests or friends, chooses to live a life in solitude. The only person clinging around him is a young gay man named Toshiki.
Late one night, Shunsuke hears a strange song coming from the unit next door, room # 369. An old woman lives on her own in that room. Shunsuke asks the old woman to stop with the singing but she tells him, "The song resonates in the heart of a child without a home. For 400 years, I have lived to marry such a child. I shall become your wife."
Shunsuke suspects the old woman is suffering from dementia and decides to verify her identification at the town office. But he learns that there's absolutely no information available. The only clue he could use to trace her back was the song she was singing.
Hearing her song through the walls every night agitates Shunsuke but eventually even leads to a change of heart within Toshiki. And one day, Toshiki disappears from Shunsuke's life. But information on the woman finally surfaces claiming that she may be the last living person who knows the whereabouts of a legendary and hidden fortune amounting to 50 million dollars. People start to appear claiming their kinship with her.
Just as Shunsuke thought the old woman leaving the apartment would lead him to reconnect with Toshiki, he learns about Toshiki's suicide, brought on by his inability to cope with loneliness. Shunsuke is completely stunned and a sense of panic sweeps over him. The only person who understands his heartache is the old woman next door. For the first time in his life, Shunsuke realizes that he is "loved."
But even the old woman, the first person Shunsuke opened up to, was approaching her death through cancer. Completely unaffected by the dismay and disbelief of others, Shunsuke decides to apply for a marriage registration with her. People begin to whisper that signing the marriage certificate was for inheriting her fortune.
Shunsuke begins to learn about the old woman's past; about how she had eloped and fled her home, about how her child starved to death during the war. Her husband had headed for war but his death was never officially confirmed. And as penance for her sins, she led a hidden life. In the present, she laments Shunsuke's suffering through her singing, which she did in anticipation of her own death.
Shunsuke had also been abused by his parents, then became an orphan after being abandoned. A mother who had lost her child and an orphan who was deserted by his parents. The two come to share a strong bond that was neither a husband-wife nor a parent-child relationship.
Shunsuke begins to investigate a fact that was most important to the old woman. He traces down her husband's whereabouts and the two visit him. After returning from the war and living to his old age, he was now suffering from dementia and couldn't recognize anyone. Shunsuke holds Setsu as she breaks down from life's grimness brought on by a void of 60 years.
At his workplace, Shunsuke's marital status had become a serious issue as they suspected him of marrying an elderly in pursuit of her fortune. Just presenting the facts didn't help to have Shunsuke's case understood, but he takes to a public hearing to clearly state his intentions. The bureau's stance remains unchanged and announces his transfer.
A year later, the old woman passes away. Once again, Shunsuke loses his family. But this time, he takes the love he had inherited from her and decides to adopt an orphan, promising himself to rebuild his family.