The culprit is Fumie Masuko. She faked the Nue by putting fake fur on the dog and used the fired she created with metallic sodium and a painting of the wall of the house to create an optical illusion. Fumie admits she killed the two victims because Charles Abel was her husband and the two didn't save him back then. Heiji also reveals that Hajime is the son of Fumie and Charles, but Fumie denies it, despite Hajime looking similar to both her and Charles.
Heiji tries to confess his feelings for Kazuha, but is disturbed by the Japan Air Self-Defense Force landing nearby on the orders of Yokomizo to capture the Nue and secure civilians. Surrounded by soldiers, Heiji decides not to confess. Some time later, the high-school student Momiji Ooka reads about the case on her smartphone and calls Heiji her future husband.
The instructions to open the box were hidden on the Nikujaga recipe page of one of the cook books. Kimika never needed to read those, since she learned her husband's favorite Nikujaga recipe from her mother-in-law.
