Grade schooler Yuuta Tomonaga stumbles upon Deckerd, a humanoid robot under construction by the Japanese police, built to fight advanced forms of crime. Yuuta's constant contact with Deckerd gives the robot a "heart," or personality; when Yuuta is recruited as the "boss" of the "Brave Police" as a result, a true human/robot partnership occurs.

After Jinta and Suzune ran away from their home as children, they were taken in by Motoharu—the virtuous sentinel of Kadono Village—and his daughter, Shirayuki. The settlement was ruled by its shrine maiden, Itsukihime, whom only a select few were allowed to interact with. When Itsukihime passed away, Shirayuki was appointed in her place. Wishing to stay in contact with her, Jinta worked diligently for years and was finally selected as the next sentinel. As the protector of the village, Jinta's duty is to eliminate any threats. One day, he encounters a demon in a nearby forest. At the end of the battle, the demon makes a proclamation about the destined ruler of all demonkind. He attaches his severed arm to Jinta, turning him into an unaging demon man. By the time Jinta comes to, Kadono is on fire. Suzune has transformed into the demon from the divination, and due to a series of misunderstandings, she murders Shirayuki and flees. Devastated, Jinta cannot forgive Suzune for her actions; he departs on a journey to find her. As Jinta travels around Japan across the eras, he endeavors to protect as many people as possible from aggressive demons. In his quiet moments, he contemplates what he should do when he once again faces his childhood friend.

The balance of the world is deeply shaken when malevolent gods known as the Four Perils break their seals and disappear together with eight million minor deities. In the absence of gods, lands become barren and famine looms in the country of the Eight Immortals. In order to restore the fragile harmony, kashi—gifted dancers and singers—must hunt the missing gods in other realms, use their performance skills to charm the deities, and seal them using the power of their rosaries. Although a master dancer, Ichiyou is ridiculed by his peers for his lack of singing talent. Nonetheless, the young kashi is tasked by the Eight Immortals to bring back the Four Perils before the world plunges into complete chaos. Helped by his familiars Tenkou and Kaka, Ichiyou embarks on the perilous mission given to him, with his search leading him to modern-day Japan. If he hopes to save the world, Ichiyou must also uncover the truth behind the Four Perils' disappearance—but this event seems to be disturbingly intertwined with his own past.

A great war occurred in the 20th century between humans and monsters. Since that time, the monsters rule the world in fear but keep relatively hidden from public view. A boy named Bokka ponders the past and wonders what became of the Meros Warriors who defended the world so bravely against the demons. He soon meets Kurofune and learns of the power of the Meros and the I-bar machines they ride in battle. Warriors are the only ones who can see and hear the Melody of Oblivion, a phantom girl hidden away waiting to be rescued and be the savior of mankind. During a battle between Kurofune and a demon, Bokka discovers something only his wildest dreams could possibly imagine...he too can hear that melody. Throughout his journeys, Bokka meets many monsters and their agents, friends, and companions as he discovers the true extent of his new powers. He must continue to battle evil in the hope of releasing Boukyaku no Senritsu and free a world that has forgotten its once beautiful melody.