Associate Professor Akira Takatsuki's Conjecture, Vol. 4 (novel)

It’s spring, and Naoki is now a second-year college student. As usual, he’s working as Professor Takatsuki’s assistant. One day, Takatsuki receives a request from a woman working in an architectural firm.

What’s the truth behind the creepy things happening in the office related to the number four? In June, a weekly magazine publishes an article about the appearance of a mermaid, causing quite the stir. The fervor about mermaids heats up even in Takatsuki’s lecture. On such a day, a dapper British gentleman wanders onto campus—and he’s Takatsuki’s uncle?! Unexpectedly, the man tells Naoki about Takatski’s sorrowful past.

Then on a day off, they go investigate the area where the mermaid appeared. There, they meet a boy who tells them that his mother turned into a mermaid.