"Darker than Black - Kuro no Keiyakusha: Gaiden" serves as a two-episode OVA that bridges the events between the first and second seasons of the main series. The narrative centers on Hei, also known as BK-201, a Contractor who possesses supernatural abilities but must pay a personal price for using them. After the events of the first season, Hei goes into hiding, distancing himself from his former allies and the Syndicate, a shadowy organization that manipulates Contractors for its own ends.

The story begins with Hei living under the alias "Li Shun Sheng" in a small town, attempting to lead a quiet, ordinary life. However, his past catches up with him when he encounters a young girl named Suou Pavlichenko, who is being pursued by dangerous forces. Suou is revealed to have a connection to Contractors and the mysterious Hell's Gate, a spatial anomaly that grants individuals supernatural powers. As Hei becomes entangled in Suou's plight, he is forced to confront his own identity and the moral complexities of his actions as a Contractor.

Throughout the OVA, the narrative explores themes of redemption, identity, and the consequences of wielding power. Hei's internal struggle is juxtaposed with the external threats posed by the Syndicate and other Contractors, creating a tense and introspective atmosphere. The story also introduces key elements that set the stage for the second season, including the emergence of new characters and the expansion of the series' lore surrounding Contractors and their abilities.

The OVA maintains a focus on Hei's character development, highlighting his evolution from a detached, mission-driven individual to someone grappling with the weight of his past decisions. Suou serves as a catalyst for this transformation, embodying the innocence and vulnerability that Hei seeks to protect. The interplay between the two characters drives the emotional core of the narrative, while the action sequences and strategic confrontations with adversaries maintain the series' signature blend of suspense and supernatural intrigue.