Fujigasaki Miho, a student at a girls' school, lost her mother, an oceanographer, a few years ago and was living with her father. Her father was the captain of the cruise ship Northern Light, but after her mother's death, he stepped off the boat.

However, Miho noticed that her father wanted to return to the ship someday. When an old coworker urged him to return, she offered a hesitant father one final push to do it. At that time, Miho was already able to do more or less all the domestic chores. His return was then arranged. After some time as first mate, it was decided that the father would officially return to the captain position. It would occur on the anniversary of the Northern Light's 200th cruise.

At that time, Miho's close friend, Moribe Jun, gave her a suggestion. Since she was hired for a short-term part-time job on that anniversary cruise, they could have worked together. Miho decided to accept the offer because it would only last for the summer holiday and Jun would also be present. It was therefore determined that she would go on the cruise, which would mark her father's return to his role as the captain of the ship.

The time of the summer holiday has come. After a few days of induction courses, the voyage has finally begun. While struggling to do the unfamiliar job, the protagonist made friends with different people, including Kurusu Nonoka, an idol who was in charge of events on the ship; Otowa Saki, a student who was studying violin; and Mizunashi Nadia, a casino dealer—and was spending fulfilling days.

This summer vacation will become a memory for the rest of her life... 
Miho was confident of it, but she had not imagined in her wildest dreams that a nightmare awaited her beyond.

[Translated from the official site]