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, in his second life he has gained the 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!

When aliens known as the Oni threaten to invade the Earth, they promise to leave under one condition—a randomly-chosen human must win a one-on-one game of tag against Lum, the beautiful daughter of the Oni leader. The lucky person selected happens to be the lustful and unlucky high schooler Ataru Moroboshi. Given 10 days to attempt to grab Lum's horns, Ataru realizes how impossible the challenge is as he is faced with Lum's extraterrestrial powers. Motivated by a promise of marriage from his childhood friend Shinobu Miyake, Ataru manages to catch Lum off guard. He mistakenly grabs hold of her bikini top first, but he eventually achieves his true goal. Although the game is over, Lum misunderstands that she is the one whom Ataru wants to marry, and she decides to move in with him. The poor student constantly tries to shake off the clingy Lum while doing his best to reconcile with his desired fiancée. After Ataru's heroic feat results in such a disastrous outcome, it is questionable whether luck will ever be on his side.

Loid Forger, an elite spy, is warned by his handler that he may potentially be reassigned from his ongoing mission, Operation Strix. To maintain his position, he must make significant progress toward the operation's objectives, which involves having his adoptive daughter Anya earn sufficient Stella Stars to become an Imperial Scholar at Eden Academy. After learning of a cooking contest that awards the winning student with a Stella Star, Loid researches the judge's preferred dessert to help increase Anya's odds. However, perfectly recreating the judge's favorite meremere requires more than just following a recipe. Thus, the Forgers embark on a vacation to the Frigis region to try an authentic meremere. Not all goes smoothly on the trip, as the Forger family ends up entwined in a sinister plot to reignite war between the countries of Ostania and Westalis.

With his mother taken away from him and having lost everything, Lag Seeing is now a letter whose delivery has been assigned to Gauche Suede, a Letter Bee. Despite their troubling start, the two of them become friends, leading Lag to realize what his aim in life is: to deliver people's most important feelings in the form of letters, just as Gauche has done.