Ryou is delighted to be reincarnated into the fantastical world of Phi, where he thinks he'll get to live a quiet life learning to use his newfound water magic. Going with the flow here, however, means something very different. Ryou is immediately pitted against the wild lands he winds up in and the slew of deadly monsters that call the remote subcontinent home. You'd think he'd forget about taking it easy when he's stuck fighting for his life, but lucky for Ryou, he's naturally optimistic, clever, and blessed with the hidden "Eternal Youth" trait. Twenty years pass in the blink of an eye, and each encounter along the way pushes him one step closer to the pinnacle of human magic. Little does he realize that's only the opening chapter of his tale. A fateful meeting soon thrusts Ryou to the forefront of history, forever changing the course of his life... Thus begins the adventures of the strongest water magician the world has ever seen—who also likes to do things at his own pace!

Seven years after humanity was devastated by a major disaster, a beacon of science and prosperity rises in Kamitsubaki City. But lurking in its shadows are Tesseractor, monsters born from human darkness. Only the Witchling, five girls with voices that wield magic, stand against them. With every note, they fight, not just to protect the peace of the city, but to heal a broken world.

The story centers on Gen Kinokura, a devoted solo camper who treasures his solitude. During one of his usual camping trips, he unexpectedly crosses paths with Shizuku Kusano, a total camping novice. What starts as an accidental encounter evolves into an unusual partnership, as the two embark on what can only be described as "solo camping for two."

Reeling from her recent breakup, Momo Ayase, a popular high schooler, shows kindness to her socially awkward schoolmate, Ken Takakura, by standing up to his bullies. Ken misunderstands her intentions, believing he has made a new friend who shares his obsession with aliens and UFOs. However, Momo's own eccentric occult beliefs lie in the supernatural realm; she thinks aliens do not exist. A rivalry quickly brews as each becomes determined to prove the other wrong. Despite their initial clash over their opposing beliefs, Momo and Ken form an unexpected but intimate friendship, a bond forged in a series of supernatural battles and bizarre encounters with urban legends and paranormal entities. As both develop unique superhuman abilities, they learn to supplement each other's weaknesses, leading them to wonder if their newfound partnership may be about more than just survival.