"Master of Mosquiton '99" is a six-episode OVA series that blends supernatural action, comedy, and romance. The story centers around Inaho, a high school girl who works part-time at a detective agency. Her life takes a dramatic turn when she encounters Mosquiton, a centuries-old vampire who has been sealed away for 666 years. Inaho accidentally awakens him by drinking his blood, which binds them together and grants her partial vampire powers. Mosquiton, seeking to regain his full strength, embarks on a quest to find the "O-Part," a legendary artifact said to grant immortality.

The narrative unfolds as Inaho and Mosquiton travel across various locations, pursued by rival factions also seeking the O-Part. These include the powerful vampire organization known as the "Black Lodge," led by the enigmatic and ruthless Alucard. Along the way, they encounter allies and adversaries, including other vampires, supernatural beings, and humans with their own agendas. The dynamic between Inaho and Mosquiton evolves from reluctant partnership to a deeper connection, as they face challenges that test their resolve and trust in each other.

Themes of immortality, power, and the consequences of seeking eternal life are explored through the characters' interactions and the unfolding plot. The series balances action-packed sequences with moments of humor and emotional depth, particularly in the evolving relationship between Inaho and Mosquiton. The OVA concludes with a climactic confrontation that ties together the central conflict while leaving room for interpretation regarding the characters' futures. The concise runtime and episodic structure make it a fast-paced and engaging supernatural adventure.