The theft case is actually a ruse for an tragic case of blackmail, in which Shuko Kumado was made an unwilling victim by Naomi Senda. Two years previously, Shuko had been seen with a strange man, who was actually Kotani Yasunobu, an old high school classmate of hers. Although Shuko's marriage with Soichi had started with mutual love, Shuko had gotten doubts about it, but was unwilling to divorce in order not to disappoint her parents, who were overjoyed at her union with a member of such a prestigious family. One month ago, Katoni had asked her to go with him to South Africa, but since Shuko was pregnant with Kazuma, she decided not to elope in order to secure her child's future.
But as they left their meeting place at the Tottori Sand Dunes, they were spied upon and photographed by Naomi, who used this information to blackmail Shuko. In fear of having her child taken away by her family-in-law, Shuko paid Naomi to keep the "affair" secret; but when Naomi had learned from the news that Yasunobu had died in an accident, she decided to raise her demands. Facing the fear that she couldn't procure the ever-mounting hush money anymore, Shuko decided to kill Naomi.
In order to conceal her intentions, she planned the fake robbery and ransoming of the incense burner along with Naomi, who would agree to cease blackmailing her once the ransom money for the artefact was in their hands. Naomi left Shuko tied up in the house and planted the broken urn and the keychain incriminating her as the culprit after they had established Naomi's fake alibi, and they started the fight to throw off suspicion that they were accomplices in this caper. Afterwards, Shuko would lure Naomi to a steep cliffside at the coast and cast her into the sea, then claim that this was done in self-defense. However, the immediate execution of her plan was foiled because of Iwao Kumado's refusal to let her leave the house while the incense burner was missing; hence, she returned it.
The decisive evidence was the delivery Shuko received at the house door during the family meeting. The delivery service man had acutally attempted to deliver the package at 12.00 o'clock, while Shuko and Kazuma were alone in the house. However, since no one answered the door, the delivery man intended to return with the package some other time; but he got a phone call to re-deliver right away. In fact, Shuko was at that time already tied up, and while the clock had been reset to provide a fake alibi for Naomi, the unplanned arrival of the delivery man upset that vital detail. Therefore Shuko was in a hurry to take the mail to hide the fact that the package has arrived as a re-delivery.
While Shuko couldn't execute her murder plan, the fact that she committed a fake robbery and would face the Kumado clan's wrath as a result cannot be dismissed that easily. However, Soichi - who has followed his wife - unexpectedly intervenes, having decided to cut his ties with his family, leave Junji in succession as the clan's head, and make a new beginning with Shuko and his son once she gets out of jail.
