One morning, Seiji runs into Kounosuke while out on a walk. The two young men discuss recent affairs on Ryogoku bridge, but Kounosuke suddenly loses his balance and falls in the river, accidentally pulling Seiji with him and losing his precious seal box, Kogare-kou, that has been passed down in his family for generations. Kounosuke does his best to look for it, but to no avail. Meanwhile, another young man appears to have found Kogare-kou and demands a considerable sum for its return. With Kounosuke facing being disowned by his family if he cannot get the object back, Seiji and Oko do their best to help him raise the money with the help of the tsukumogami.