In modern-day London, a secret war is brewing in the night ­ a war in which humanity is only a pawn. A mysterious agency called the Hellsing Organization, deploys in the shadows to protect ordinary mortals from the undead legions who would prey upon them. Led by Integral Wingate Hellsing, leader of the Protestant Knights Hellsing Institution, and the great granddaughter of Abraham Van Hellsing, (From Bram Stokers Dracula), the Institution has a secret weapon at its disposal: an ancient and unbelievably powerful vampire named Arucard. This rogue vampire can deal with any problem but one: Seras Victoria, a cute and naive new operative whom Arucard transformed into a vampire in order to save her life. Her confusion about the vampire lifestyle is a source of conflict between her and Hellsing, and things don't get easier when a wave of enemy activity sends her and Arucard into action.

Recently there have been a surge in undead activity. Vampires rampage through the countryside, bloodthirsty corpse-ghouls infest the London subway systems and terrorist attacks destroy vehicles and buildings all over the city. Behind the attacks are 'artificial vampires' created by the mysterious FREAK chips, which can turn any normal person into a vampire, and which are circling the London underworld like a new designer drug. Nobody knows who's producing the FREAK chips, and Hellsing has its hands full with the attacks, but their trouble has just begun: The Roman Catholic Church's secret vampire-hunter squad, Iscariot Division 13, has begun encroaching on Hellsing's territory, using the London attacks as an excuse to wage war with Hellsing itself. Iscariot's agent Alexander Anderson, a murderous priest with a few deadly powers, trips up Hellsing at every turn and vows to destroy every vampire, Arucard included.