On a full moon night the beautiful and sinuous Lady Anthea Recluse hurriedly enters the library of a lonely English manor house from the grounds outside, wipes her mouth, and starts obsessively looking through the books. Meanwhile, her crotchety older husband, Lord Roderick Recluse, is looking for his dog, Bodger. Gunther, the strangely lupine, oddly-accented manservant, announces the arrival of the handsome Scottish Chief Inspector Cottage. He has grim news: Bodger’s corpse has been found in the fens, apparently savaged by a gigantic cat-like animal. 

Alone together, Anthea and Gunther fall into sibling squabbling in their native, vaguely Eastern European accent– for they are brother and sister from the obscure land of Batvia. Music plays whenever the name Batvia is mentioned, and they are compelled to dance the strange folkloric dance of their homeland. Worse, they are under an ancient familial curse: if they’re exposed to the light of the full moon they turn into ravening beasts – Anthea a werepanther, Gunther a werewolf. The curse was cast 499 years ago; when 500 years have passed they will become full-time lycanthropes forever. They’ve come to England to find The Book of Spells which contains the counter-spell. Anthea, pretending to be English, has married Roderick in order to have access to his library, as Roderick’s ancestor stole the Book from Batvia years ago.  Anthea and Gunther have tried to refrain from their bloodthirsty propensities while on their quest, but Anthea/panther has apparently lapsed and done in the dog. Jealous, Gunther steps out into the full moon’s light; we hear a wolf’s howl.

The next morning the ravaged body of Dora the cook is discovered. When Roderick, Anthea and Gunther gather, Roderick sees the crest of Batvia on Gunther’s dressing gown. He realizes with horror that the legends in the ancient book are true, and that his wife and man-servant are murderous wer-beasts. He eyes a stuffed elk head before collapsing. Behind it, Anthea and Gunther find…The Book! Anthea discovers that the counter-spell must be cast in Batvia, on the first full moon of the coming New Year. They must leave at once! Roderick, however, has recovered sufficiently to accuse them of transforming into wild beasts and murdering Bodger and Dora. Anthea and Gunther divert the dubious, still devilishly handsome, and now gun-wielding C.I. Cottage, and escape into the night.

A month later, Anthea and Gunther are in a windswept, high mountain pass, crossing the border into Batvia. It’s just before sunset on the last day before the turning into full-time beasts curse becomes irrevocable. Roderick and Cottage enter in pursuit and surprise the siblings as they prepare to cast the counter-spell. Anthea pleads to be allowed to complete their mission. Although he doesn’t believe in this wer-beast nonsense, Cottage has a crush on Anthea and he allows it. The spell goes awry, and everyone ends up where they should: Gunther reveling in remaining a beast; Lord Roderick gleefully pursuing him; and Cottage and Anthea in each other’s arms.