Rentt Faina, a low-ranking adventurer in the town of Maalt, dreams of achieving greatness but is stuck taking on menial tasks due to his lack of skill and experience. During a routine exploration of a dungeon, he encounters a powerful dragon far beyond his capabilities. The encounter ends in his death, but instead of perishing, he reawakens as a skeleton, retaining his memories and consciousness. Struggling to comprehend his new undead existence, Rentt resolves to continue his journey as an adventurer, now with the goal of uncovering the mysteries behind his transformation and regaining his humanity.

As a skeleton, Rentt discovers that he possesses unique abilities, including the capacity to absorb the essence of monsters he defeats, allowing him to grow stronger and evolve into more advanced forms of undead. This evolution grants him new powers and a semblance of his former appearance, though he remains far from human. Along the way, he encounters Lorraine Vivie, a skilled sorcerer and scholar who becomes his ally and confidante. Lorraine assists Rentt in his quest, providing knowledge and support while also researching the nature of his condition.

The narrative explores Rentt’s dual existence as both an adventurer and an undead being, navigating the challenges of hiding his true nature while interacting with the human world. His journey involves uncovering ancient secrets about the world’s history, the origins of monsters, and the nature of divinity. As he grows stronger, Rentt faces increasingly powerful foes, including other undead creatures, mythical beasts, and human adversaries who threaten the balance of the world.

The series delves into themes of identity, perseverance, and the pursuit of knowledge, as Rentt grapples with his transformation and the implications of his undead state. His relationships with other characters, particularly Lorraine, add depth to the story, highlighting the importance of trust and camaraderie in overcoming adversity. The world-building is intricate, with a focus on the lore of adventurers, guilds, and the supernatural forces that shape the setting. Through its blend of action, mystery, and character development, "Unwanted Undead Adventurer" presents a compelling tale of an unlikely hero striving to redefine his destiny.