"Spiritpact -Bond of the Underworld-" is a supernatural anime series based on the Chinese webcomic by Pingzi. The story centers on Keika You, a young exorcist who, after a tragic accident, becomes a spirit. Following his death, he encounters Tanmoku Ki, a powerful spirit who serves as a "Spirit Exorcist" tasked with maintaining balance between the human and spirit worlds. Tanmoku offers Keika a pact: by forming a bond, Keika can continue to exist as a spirit and assist in exorcising malevolent entities. Reluctantly, Keika agrees, and the two form a Spiritpact, binding their fates together.

As the series progresses, Keika and Tanmoku navigate their partnership, which is initially strained by Keika's reluctance and Tanmoku's aloof demeanor. Together, they face various supernatural threats, including vengeful spirits and other exorcists with conflicting agendas. The narrative explores their growing bond, as well as the challenges of their respective pasts. Keika struggles with his new existence as a spirit and the loss of his human life, while Tanmoku harbors secrets about his own origins and the true nature of his duties.

The anime delves into themes of loyalty, identity, and the blurred lines between life and death. Supporting characters, such as the cheerful spirit Lan and the enigmatic exorcist Bai, add depth to the story, providing both assistance and complications for the main duo. The series balances action, humor, and emotional moments, with a focus on the evolving relationship between Keika and Tanmoku as they confront both external threats and internal conflicts. The narrative builds toward revelations about the Spiritpact itself and the greater forces at play in the spirit world, culminating in a climactic resolution that tests their bond and resolve.