Ginkgo-Colored First Love (Part One)

The Detective Boys are at Agasa's house when they happen to find a letter from Agasa's first love. Apparently they are supposed to meet every decade but was unable to because Agasa could not solve the code from the letter. The Detective Boys set out to solve it and find the "memorable place" using the hint given in the letter. The clues lead them to the zoo where Agasa see's a brown hair women.