Mermaid no Kisetsu is a PlayStation port of the PC hentai visual novel with stripped adult content. The game follows Masato Takahata, a high-school sophomore, living alone, going to school classes and working a part-time at Aqua Rouge cafe. The setting is in the near future with technological advancements, but the living style didn't change that much. One day you find a strange note with an address of a home page. You check it out only to see the ominous warning text. While you're not trying to figure out the mystery behind it, you will traverse town meeting various people, mostly girls and women, with whom you'll work on relationship to become friends with, or something more.

The core of the game is a romance visual novel with dialogues and occasional reply choices with full voice-acting for all the heroines as well as other side-characters. An additional gameplay feature is that when you're home in your room, you can check the mysterious home page, as well as home pages of the girls you meet in town, reading info about them, checking their diaries and posts. The game progresses through the days, always showing the day of week and the time of day, with ability to select your next location on the town map to progress the time, but you can only select to visit the places where other characters are, even if you avoid talking to them.