The story begins with Rondo, a fifteen-year-old illegitimate son of the Edouard Marquis, finding himself imprisoned in a gilded cage. Despite his noble lineage, Rondo lacks any magical talent, a crucial requirement for nobility in the Yugtia Kingdom. His father, the Marquis, has presented him with a final "gift": a cup containing wine and a deadly poison, with the implicit command to commit suicide upon reaching adulthood. This is presented as a mercy, a way to avoid a more brutal end. Rondo, filled with despair and anger at his fate, drinks the poisoned wine.
Unexpectedly, Rondo survives. Instead of death, he experiences intense heat on his back, discovering a black dragon-shaped magic crest, a sign of magical talent. He realizes the poison, rather than killing him, has awakened his latent poison magic, a rare, outside-attribute magic that he was unaware of. His survival is due to the poison itself being the catalyst for his magic, allowing him to neutralize it and gain the ability to use "weakening poison."
Fearing discovery and further punishment, Rondo devises a plan. He uses his newfound poison magic on himself, inducing a state of near-death. His brother, Felix, who harbors disdain for Rondo, confirms his apparent death and orders the body disposed of. However, Rondo had faked his death, and after being placed in a grave, he revives.
Now a fugitive, Rondo flees to the Anastasia Duchy, seeking to hone his poison magic and avoid his family. He registers as an adventurer, specializing in poison-related skills, and learns that the Duke's daughter, Marie, has been poisoned. The Duke is desperately seeking a cure, offering rewards for any potential remedies. Rondo, seeing an opportunity, offers his services, requesting protection from the Edouard family in exchange for curing Marie.
His unique abilities, particularly his dragon-shaped magic crest and his poison magic, impress the Duke and his physician. Rondo reveals his past and his newfound power, convincing the Duke to grant him protection. He is tasked with finding the culprit behind Marie's poisoning. Rondo undergoes a trial, ingesting the very poison that afflicted Marie, a potent dragon-type poison. This act further enhances his poison magic, granting him the "Dragon Poison Tree" and the ability to use various poison-based spells, including the powerful "Poison Dragon."
Rondo, now a guest of the Duke, begins his training in earnest, accompanied by Marie, who also fears for her safety after being attacked by a trusted servant. They travel to a secluded villa where Rondo can practice his magic without restraint. He discovers his poison magic can now be used as a direct physical attack, capable of destroying trees. He also learns he possesses a vast amount of magical power, inherited from his father.
His training progresses, and he begins to fight monsters, honing his skills with different poisons and developing combat strategies. He learns to visualize his opponents' HP and use his poisons effectively. His ultimate goal is to become strong enough to protect himself and Marie, and to uncover the conspiracy behind the attack on the Duke's daughter.
