Reincarnated as a cheat in another world, a laid-back and powerful slow life ~An S-rank old man who was expelled from the guild becomes unrivaled with his useless skills and has his engagement broken off by a hero, a demon king, a sage, a saint, a slave, a magician, an alchemist, and a villainous young lady~

The story begins with Gaku, a weary-looking man in his thirties, sitting in a rundown bar with Karen, the young-looking proprietress. Gaku, who claims to be on the run, explains his past as an unintentional ability user. In this world, magic consumes mana, while abilities are special powers used without it. Unlike magic, which can be learned by anyone, abilities are unique, and ability users cannot use magic for unknown reasons. Gaku discovered his ability to read minds when drunk, which led him to become an adventurer. He registered as an S-rank adventurer, a rank he found ironically low, as it only allowed him to take on menial tasks.

His early adventuring career was disastrous. He joined a low-ranked party but was quickly expelled after a drunken goblin-slaying attempt where his mind-reading ability proved useless against monsters. A turning point came when, after getting drunk and causing a scene, he was subdued by Cynthia, a woman who would later be known as the Hero. He joined Cynthia's small party, which included her cat-eared demi-human servant, Rumia, on a quest to defeat the Demon Lord.

Their journey led them to a city where they accepted a quest from a famous alchemist, Veronica, to retrieve a monster's horn from a dangerous dungeon. Gaku discovered his mind-reading ability was effective against intelligent monsters, allowing him to anticipate their moves and contribute significantly to their victory. However, a drunken night led to Gaku waking up next to a naked Veronica, resulting in a five-year engagement to postpone the issue.

The party then traveled to the kingdom of Lakeud, where Gaku encountered Princess Seila, who had been transformed into a giant toad-like monster. Gaku's ability allowed him to understand her plea for help, and he promised to save her and her kingdom from a treacherous minister. They sought the aid of Maya, a witch, who, unable to break the curse herself, directed them to her master, the sage Melia, on a remote island.

Another drunken misadventure led Gaku to wake up next to a naked Maya, resulting in a four-year engagement. Upon reaching Melia, they learned she could break Seila's curse. Their journey continued to the port town of Sanmarine, where they were tasked with proving the innocence of Fiona, the Count's daughter, who was accused of distributing a dangerous drug. Gaku's ability confirmed Fiona's innocence, and they apprehended the true culprit, a port security officer. This led to another drunken incident, resulting in Gaku waking up next to a naked Fiona and a four-year engagement.

They finally reached Melia's island, where they learned of a former hero who might know how to defeat the Demon Lord. After a perilous sea voyage, during which Gaku's mind-reading proved crucial in defeating a Kraken, they arrived at the island. Melia, an elf, agreed to help them break Seila's curse. However, another drunken night led to Gaku waking up next to a naked Melia, resulting in a four-year engagement.

After breaking Seila's curse and defeating the minister, the party returned to Sanmarine. Gaku, having avoided another encounter with Fiona, set sail for the elf village. During the journey, they encountered a giant Kraken, which Gaku's mind-reading ability helped them defeat. Upon reaching the elf village, they were welcomed by Melia's kin. Another night of drinking led to Gaku waking up next to a naked Rumia, resulting in a three-year engagement.

The group then sought out a former hero who lived in a secluded area. They learned that the Demon Lord's castle was located within a volcano. After a challenging dungeon crawl filled with mechanical dolls that Gaku's ability couldn't read, and a near-fatal injury to Rumia, they finally reached the Demon Lord's castle. Gaku, having realized his invisibility to powerful enemies due to his lack of magic, decided to confront the Demon Lord alone.

He entered the castle, intending to sacrifice himself, but was interrupted by Cynthia, who confessed her love and proposed marriage if they defeated the Demon Lord. Gaku then joined forces with Cynthia, and together, with the combined efforts of the party and the timely intervention of Maya and Melia, they defeated the Demon Lord. However, immediately after their victory, Gaku found himself waking up next to a naked, unknown woman, leading to another engagement.

The narrative concludes with Gaku explaining his predicament to Karen, the bar proprietress, revealing his numerous engagements and his attempt to escape his responsibilities. Karen advises him to apologize, and Gaku, realizing he had forgotten the crucial step of apologizing, leaves the bar with a renewed sense of purpose.