In a near-future Japan, a secretive and highly popular trading card game known as LuvSlave has emerged as a rite of passage for adolescent boys undergoing their sexual awakening. The game is not played with cards alone; it requires a male Master to form a contract with a female Slave, typically an older woman or young lady. Battles unfold as the Master uses cards to command his Slave to wear revealing outfits or perform specific techniques, but with a crucial rule: every action can only be activated with the explicit approval of the Slave, theoretically making it a test of trust and affection between the pair.

The central character is Yuto, a shy and self-described boring boy who is interested in the game but too timid to approach a girl to be his Slave. On his birthday, his caring older sister Akemi gives him a LuvSlave starter kit and, understanding his predicament, volunteers to enter into a contract with him as his first Slave. Akemi is initially reserved, but taking on the role of Slave awakens a newfound and enthusiastic side of her personality. Their relatively peaceful introduction to the game is shattered by Rettsu, a veteran and ruthless player who easily defeats Yuto in their first match. Seeking to win his sister back after a loss, Yuto must navigate the complex world of LuvSlave.

A central element of the game is the NTR system, an acronym for a mechanic where a Master’s Slave can be claimed or stolen by another player, leading to intense rivalries and forcing Yuto to protect his relationship with Akemi. Following his initial loss, Yuto receives guidance from a mysterious and mature woman named Kisaragi, who is a member of the World LuvSlave Association. She enters into a temporary one-night contract with Yuto as his Slave to help build his confidence as a Master, coaching him on strategy and the deeper rules of the game. Unknown to Yuto, Kisaragi has a hidden agenda: she is investigating a series of disappearances of both Masters and Slaves who share a heart-shaped tattoo.

The principal battleground for these card games is the Underground World, a secluded space isolated from a surface world in crisis. This realm is accessible through portals and is strictly divided into gender-segregated zones. Only boys between the ages of seven and eighteen, along with their contracted Slaves, are permitted entry into the male-only areas, making it the primary setting for the series' strategic and erotic battles. Wins and losses are determined by reducing the opponent Master’s sperm amount to zero through various attacks performed by the Slaves, effectively gamifying sexual acts.

As Yuto grows more proficient, he partners with other Slaves, such as the confident and skilled Saki, and learns that one of the unspoken but critical rules is the taboo of using family members as Slaves. Near the end of the first major narrative arc, Yuto is challenged by the villainous Taiyou, who reveals a shocking connection to Yuto’s past and forces a climactic battle that pushes the game’s rules to their absolute limits. The storyline continues to explore Yuto’s development as a player while the looming threat of the disappearances investigated by Kisaragi suggests a darker, more dangerous layer beneath the surface of the game.