An old but pretty woman comes in Kogoro's office, asking him to find the address of her first love. Kogoro gets very interested in her,and Conan quickly finds out the address. Conan is also suspicious about this woman because he noticed that she kept telling lies-- saying that she wasn't married when her ring finger was thinner and saying that she never cooked when she could recognize different kinds of fish just by looking at the gills. But when they arrive, they find an open door. And the open door revealed a bloody crime scene-- the woman's first love lies dead on the floor.

Conan, Ran, Heiji, Kazuha, and Kogoro go to the Isle of Mermaids at the request of a client.