When a hole opened up in the world and monsters flooded out, humanity prepared for the demon lord Rukimedes' rise once again. However, even the gravity of this situation cannot impede the antics of straight man hero Alba Frühling and sadist soldier Ross. Ross, trapped within a plane between two worlds due to Januar Ein's magic, encounters the demon lord taking a nap and looking far too comfortable in his confinement. Simultaneously, Alba, once again in prison, has been sidelined in favor of his eccentric companion's hijinks. Although separated, Ross and Alba continue their attempts to prevent the demon lord's resurgence, but unbeknownst to them, a ghost from Ross' past may just threaten their quest's success.

During his summer vacation, middle school student Nagasumi Michishio travels to the Seto Inland Sea. One day, while swimming at Mio Sun Beach, his leg suddenly cramps. No one is close enough to notice his desperate screams for help, and so he sinks into the ocean, where he is left to drown alone. Just as he loses consciousness, however, a mermaid appears and saves his life. That night, Nagasumi is visited by his savior, a girl who introduces herself as Sun Seto—a mermaid from a yakuza family. As it turns out, under mermaid law, a mermaid whose identity is revealed to a human must be punished by execution. To avoid this harrowing outcome, the Seto family propose a solution: Nagasumi must marry Sun or die at the hands of Gouzaburou, Sun's father and boss of the Seto clan. Faced with no other option, Nagasumi takes her hand in marriage. Now, the newlyweds face the difficult task of keeping their relationship secret. Between Gouzaburou's unending attempts on Nagasumi's life and the eccentric antics of a slew of antagonists, a genuine and innocent love blossoms between the pair as they adapt to their new life.

On his way to school, Hiiro Ryuugasaki hears a strange voice coming from his storage shed. As he searches the place, he discovers a smartphone with the popular digital card game Shadowverse already installed. Under the assumption that his grandfather has finally decided to buy him a smartphone after countless requests, Hiiro finds nothing amiss. Later that day, Hiiro receives concerning news regarding his classmate, Mimori Amamiya. She has lost as yet another victim of Takuma Kibakura, a fellow Shadowverse player who bullies his opponents by seizing their phones after he defeats them in battle. Determined to retrieve Mimori's smartphone, Hiiro enters an exciting new world, filled with intense matches and fierce Shadowverse opponents—including the formidable Lucia Yonazuki.

Shamans are extraordinary individuals with the ability to communicate with ghosts, spirits, and gods, which are invisible to ordinary people. The Shaman Fight—a prestigious tournament pitting shamans from all over the world against each other—is held every five hundred years, where the winner is crowned Shaman King. This title allows the current incumbent to call upon the Great Spirit and shape the world as they see fit. Finding himself late for class one night, Manta Oyamada, an ordinary middle school student, decides to take a shortcut through the local cemetery. Noticing him, a lone boy sitting on a gravestone invites Manta to stargaze with them. Realizing that them refers to the boy and his ghostly friends, Manta flees in terror. Later, the boy introduces himself as You Asakura, a Shaman-in-training, and demonstrates his powers by teaming up with the ghost of six-hundred-year-old samurai Amidamaru to save Manta from a group of thugs. You befriends Manta due to his ability to see spirits, and with the help of Amidamaru, they set out to accomplish You's goal of becoming the next Shaman King.