When a salaryman is returning to work on a Monday after his holidays, a buxom high school girl named Ai bumps into him and knocks him over causing him to be taken to hospital. She asks him to act as a type of bodyguard to protect her from the other passengers aboard the crowded train and so their friendship begins.

One Monday morning, a Junior office woman offers to assist her senior, the Salaryman who is struggling to meet his deadline, and they end up working until dawn.

One day, Salaryman is scolded by his Manager for not satisfying his quota. He sees a pamphlet for the Tawawa Sports gym and thinks about visiting it. Meanwhile his manager visits the gym on the weekend and after being given a tour, he joins up. On the following Monday, Salaryman's manager advises him that "result isn't everything, the progress is" and suggests that he join a gym.

One Monday morning while Ai gives the salaryman another button, this time off her part-time job uniform, he thinks about her neat appearance and her popularity.

One Monday morning, Ai tells Salaryman that she skipped breakfast because she is due for a regular medical check up at her school. She is concerned that her large breasts make her a comparatively heavy for her height.

A female employee asks if Junior wants to go on a blind double date. She is reluctant and after she mentions her long distance boyfriend, Senior saves her by saying they have to work. One evening, after a work related drinking party, Senior carries the drunk Junior back to her apartment home on his back and she reveals that she actually has no boyfriend but uses it as an excuse to avoid unwanted romantic advances from men.

One Monday morning in Summer, Ai tells Salaryman that she's going to spend her summer vacation with her cool and popular best friend. Salaryman complains that unlike students, workers barely get a day off, but when Ai send him a photo of herself in a new swimsuit saying that her scheduled trip with her friends isn't taking place due to a storm and asks him to take her to the sea over the weekend instead. He decides to take the weekend off from work.

One Monday morning, Ai notices that Salaryman has a toothache and advises him to go to the dentist. His appointment follows his manager, who praises the painkiller the dentist used.

One morning, Salaryman visits Ai-chan at the bakery café where she works part-time job as a waitress. Although quite businesslike while on duty, AI arranges to meet him after work and gives him her waitress uniform with some missing buttons.

Ai is having trouble running in the school's marathon and is intrigued why her friend in the volley ball club, who also has fairly large breasts, seems to be not bothered. Her friend then grabs and supports Ai's breasts making the running easier.

Salaryman attends his junior high school reunion and meets a girl who he missed his 'chance' with back in their student days. She has now blossomed into a buxom woman.

One Monday morning on the train, Salaryman asked what Ai did during the Christmas day, but she initially refuses to answer. Later, she reveals that she 'has taken a step into adulthood', moving from 'I-chan' to 'J-chan'.[c]