Shun Hashimoto is an openly gay aspiring novelist living in Okinawa who was abandoned by his parents after coming out to them. Mio Chibana is a reserved, orphaned high school student, often found spending his time by the sea. One day, the two meet on the beach, and Shun is instantly captivated by Mio. The days fly by as they slowly begin to grow closer until Mio suddenly announces that he has to leave for the mainland. Three years pass before a 20-year-old Mio returns to Okinawa to confess his love to Shun. However, in those three years, Shun's life has changed. Will he be able to accept Mio's feelings and make such a commitment?

On their way home from high school, Tamahi and some classmates are shocked as they witness someone fall from the rooftop of nearby apartment. Panicked and confused, they rush to the building only to find it deserted. As Tamahi wonders about this, she has the uneasy feeling they are being watched. What is the strange presence around her...

Continuing their perilous descent down the Abyss, Riko, Regu, and newfound friend Nanachi reach the Abyss' fifth layer, The Sea of Corpses. Upon arriving at the research station known as Idofront, the main trio encounter the mysterious Prushka, the alleged daughter of Bondrewd, who leads them to the White Whistle responsible for Nanachi's dark past. Despite the welcoming appearances of Idofront's residents, Nanachi warns the young adventurers that things are not always what they seem. With the only route to the sixth layer shrouded in mystery and Bonedrewd's schemes awaiting them, what sacrifices must be made in order to continue the journey down to the bottom of the Abyss? [Written by MAL Rewrite]