Haha dakedo Koishite Ii ka na? - Toshishita Joushi ni Kyuukonsaremashita

Rina, a single mother who was abandoned by her husband, is struggling daily for the sake of her son Sota, saying, “I don't want him to feel lonely just because he doesn't have a father."
One day, while out with Sota, a man she has never met before suddenly proposes to her...? Rina rejects the joke, but for some unknown reason, she runs into him again at work! Yohdai Kawase, a man from the head office, seems to have a reason for his proposal...
What will happen to the love between a single mother who wants to be a hero and a younger boss who has his own reasons?