Mika Yamada is a contract office worker. She is 39 years old with her 40th birthday coming soon. She lives in a small studio rental apartment in Tokyo and spends her days commuting back and forth to her office.
She used to be a full-time employee. However, when the company she was working for restructured the business because of financial difficulties, she lost her job. After job-hunting, the position she found is her current clerical job. Most of her classmates are married and have children. Mika has never intended to remain single and has gone out to many dates. However, time went by and she remained single and now she’ll be 40 years old soon. She cannot believe it.
She thought she would get married like everybody else. She hates the holiday season like Christmas when she feels more lonely amid happy families with children and smoochy couples.

She wonders what her life should look like if she responded favorably to what seemed like a marriage proposal by a sincere college classmate, if she hadn’t refused the invitation to the second date from a wealthy but older and plump bachelor she met by arranged blind date, if she hadn’t dumped a very handsome but unreliable and poor younger bachelor, or if she hadn’t quit going to the accounting class she was taking.

On her 40th birthday she receives a mysterious device, which is sent from her unknown future relative. It is a time travel machine, which enables her to go back to the past.