It’s Valentine’s Day–and while Domeki is showered with chocolates and cards from girls, Watanuki receives none. To make matters worse, he must also do the usual chores for Yûko, which includes making chocolate cake for her and Mokona, as well as the treats his boss wants to give away as gifts. But when Watanuki discovers he has a shy and secret admirer who is not quite human, he finds that chocolates can be more than just sweets.

Then, after seeing identical twin sisters pass by in the street, Yûko makes a curious remark: that there are chains that only humans can use to bind others. Watanuki meets the sisters and senses that the relationship between them is not what it seems. . . .