Warning: The following contains spoilers for The Night Beyond the Tricornered Window Episode 4, “Pitfall,” now streaming on Crunchyroll.
What makes The Night Beyond the Tricornered Window unique is its intriguing plot and worldbuilding, seemingly mundane elements and characters end up being used in unique ways. For example, Mukae and the detective Hanzawa are both introduced in very plain ways, but both turn out to serve very important purposes, both in terms of plot development and world-building. However, Episode 4’s character introduction is even more significant; it not only furthers the plot but potentially changes character dynamics in a major way.
This episode also introduces the idea of building a cursed trap, which we see Erika and the mysterious “sensei” building in front of a ramen shop. As Erika later explains, this is a place they create curses, so it's now full of negative energy that can attract the negative energy of passersby. However, as people’s negative energy is added to the pile, they also become tainted or cursed in the process. The “sensei” uses this place to collect this energy, which he calls the “savings box.”
Mikado and Hanzawa visit such a place, where many people have gone missing after entering, and Mikado becomes easily infected by the curses. Although it was established that Mikado's body is easy to possess, Hiyakawa is still surprised by the extent to which Mikado is affected.
Mikado passes out to the point of losing his heartbeat. Hiyakawa tries to revive him with a paranormal AED, but whatever’s inside Mikado is incredibly strong and even tries to take Hiyakwa’s powers. Luckily, Hanzawa has the power of “disbelieve,” and his calls awaken the real Mikado, allowing Hiyakawa to finally exorcise the power inside him.
When Mikado awakens, he suddenly remembers a box in the shape of his mother’s jewelry box and the figure that’s clearly “sensei,” who was with Erika earlier. Mikado asks his mother about the jewelry box, who she says this is the only thing that his father left her. She also gives him the pendant on top of the jewelry box as a keepsake.
When connecting all the dots, it's easy to see how Mikado’s father is very likely “sensei” and the person who possessed Mikado. On top of Mikado’s obvious visions, we know he's making a “savings box” with Erika, which is also a place that Mikado and Hanzawa visited. While Mikado is possessed, he even calls Hiyakawa a “savings box." Hiyakawa is surprised by how well-matched Mikado is to the cursed place, which means that Mikado is connected to its maker. Combining all these clues with last episode’s reference to Mikado’s father’s strange disappearance, sensei’s real identity is all but confirmed.
This ordeal inspires Hiyakawa to make similar places; he plans to curse a place and then offer to clean it up as a way to increase business. As hinted at in past episodes, Hiyakawa has a very weak sense of morality, and Mikado is the only thing keeping him from becoming a true villain.
Mikado is shocked at Hiyakawa’s indifference toward hurting others, and he decides to take charge and look inside Hiyakawa in order to figure out Hiyakawa’s true intentions. Hiyakawa allows Mikado inside his core but admits that even he doesn’t know what Mikado will see. All Mikado can see is a twisted world of darkness and emptiness. A younger Hiyakawa appears before Mikado is returned to the real world.
This is the first time Mikado actually takes charge of any situation. He's been quite passive throughout the series, and this is one reason why he’s so easy to possess. However, now that he knows how to actively use his powers, it shows that not only are Mikado’s abilities growing, but so are his concerns for Hiyakawa’s well-being. Mikado truly worries about Hiyakawa now that he sees the darkness Hiyakawa’s been living with. And the young Hiyakawa is potentially the key to unlocking Hiyakawa’s past traumas.
