"Sennen no Yakusoku" is a short anime film that centers on themes of love, loss, and the passage of time, set against a backdrop of historical and fantastical elements. The story unfolds in a world where humans coexist with spirits, and the boundary between life and death is fluid. The protagonist, a young woman named Haruki, is a spirit medium who communicates with the deceased to help them find peace. Her life takes a dramatic turn when she encounters a mysterious man named Kaito, who claims to have lived for a thousand years. Kaito is bound by a curse that prevents him from dying, and he seeks Haruki's help to break it.

The narrative explores the connection between Haruki and Kaito as they delve into the past to uncover the origins of the curse. Through a series of flashbacks, it is revealed that Kaito was once a warrior who made a pact with a powerful spirit to save his loved ones during a time of war. This pact granted him immortality but at the cost of eternal loneliness, as he outlives everyone he cares for. Haruki, who has her own unresolved grief from losing her family, becomes emotionally invested in Kaito's plight. Their journey intertwines their fates, as Haruki uses her abilities to navigate the spirit world and confront the entities responsible for Kaito's suffering.

The story is driven by the emotional weight of its characters' struggles, particularly their shared experiences of loss and the desire for closure. Haruki's determination to help Kaito is paralleled by her own need to come to terms with her past, while Kaito's longing for an end to his endless existence highlights the burden of immortality. The film's pacing is deliberate, allowing the audience to absorb the depth of the characters' emotions and the intricacies of the world they inhabit. The resolution of the curse ties together the themes of love and sacrifice, as both Haruki and Kaito must make difficult choices to find peace. The narrative concludes with a poignant reflection on the transient nature of life and the enduring power of human connections.