Thirteen years before the story begins, a fearsome monster known as the Poison Queen attacked the Jade Kingdom. A party of adventurers managed to defeat her, but one of their members was struck by a terrible curse. To survive, she was forced to pass this curse down to her unborn son. This child is Caim Halsberg. Since birth, Caim has been a cursed boy whose entire body is invaded by poison, making him an object of hatred and fear. He is despised by everyone, including his own father and twin sister, who reside in the family mansion while Caim is forced to live alone in a hut in the forest. The only person who shows him kindness is Tea, a loyal maid working for his family.

Caim eventually learns that the source of his affliction is a calamity named the Poison Queen, sealed within him since birth. When a certain doctor arrives with a potential cure, Caim is forced to confront this entity. He risks his life to fight against the Poison Queen and, in doing so, successfully inherits her vicious abilities. He awakens as the Poison King, the strongest being who can manipulate a wide variety of toxins. The process also triggers a physical change, aging his body by approximately five years into that of a young man.

Having gained control over his powers, Caim decides to sever ties with his abusive family, leave his hometown behind, and live freely. He is also a naturally gifted martial artist, a talent inherited from his father, which he now unleashes alongside his new poison abilities. However, Caim soon discovers that his toxins have an unexpected side effect: a powerful aphrodisiac quality. During his journey, he inadvertently arouses the lust of the beautiful women he encounters and helps. This leads to the formation of a harem around him, including the devoted maid Tea, a noble princess who becomes an enthusiastic hedonist, and other beauties drawn to his power and charms.

As the narrative progresses, Caim and his companions become embroiled in the political turmoil of the Garnet Empire. After negotiations break down with the first imperial prince, Arthur, Caim and his group are marked as enemies of the nation and forced to flee the capital. Following the suggestion of Millicia, one of his lovers and a princess of the empire, they decide to head east to seek an alliance with her other brother, the second imperial prince Lance. On their journey, they pass through a county suffering from a wyvern outbreak. Moved by Millicias compassion for her people, Caim resolves to use his power for her sake and help save them. Meanwhile, a beautiful assassin targeting the imperial family sets her sights on Millicia, leading to further conflict as she attempts to complete her mission without falling under Caims spell.