After getting caught up in an accident, Ryou Mihara is reincarnated in the world of Phi as a water magician. Waking up in a remote forest, he sets out to achieve his ideal slow life by living off monsters and refining his magic. Unbeknownst to Ryou, decades pass without him realizing due to his hidden trait, "Eternal Youth."

One day, Ryou stumbles across a shipwrecked adventurer named Abel. As Abel requests Ryou to accompany him to the town of Lune, they embark on a journey through mountains infested with monsters. Upon arriving at the town and registering as an adventurer, Ryou's powerful water magic and equipment are unlike anything anyone has ever seen before. Although his prowess piques the interest of many, Ryou is eager to begin his own adventure and hone his magic even further!