"Hellsing" centers around the Hellsing Organization, a secret British group dedicated to combating supernatural threats, particularly vampires and other undead creatures. The organization is led by Sir Integra Fairbrook Wingates Hellsing, a strong-willed and authoritative figure who inherited her position after her father's death. Under her command is Alucard, a powerful and enigmatic vampire who serves as the organization's most formidable weapon. Alucard's origins are deeply tied to the Hellsing family, and his loyalty to Integra is unwavering, though his methods are often brutal and unorthodox.

The narrative begins with the resurgence of a mysterious group known as the Millennium Organization, a Nazi remnant that seeks to create an army of artificial vampires to wage war on Britain. As the Hellsing Organization investigates these threats, they encounter Seras Victoria, a young police officer who is turned into a vampire by Alucard after a near-fatal encounter with a rogue vampire. Seras struggles to come to terms with her new identity and powers, gradually evolving into a key member of the team.

The conflict escalates as the Millennium Organization launches a full-scale assault on London, unleashing chaos and destruction. Alucard, Seras, and the Hellsing forces engage in intense battles against the artificial vampires and their leader, the Major, a deranged and calculating strategist. The series explores themes of power, loyalty, and the nature of humanity, particularly through Alucard's complex relationship with his own monstrous identity and his role as a protector of humanity.

The story is marked by its dark and violent tone, with graphic depictions of combat and moral ambiguity. Key characters include Walter C. Dornez, a former Hellsing operative and master assassin who wields razor-sharp wires, and Alexander Anderson, a fanatical priest and rival of Alucard who leads the Vatican's Iscariot Organization. The interplay between these characters and their ideologies adds depth to the narrative, culminating in a climactic confrontation that tests the limits of their strength and resolve. The series concludes with the resolution of the Millennium conflict, though the cost of victory leaves lasting impacts on the characters and the world they inhabit.