In order to keep one of his mistresses, Celia (Polly Walker), at arm's length, King Charles II (Sam Neill) asks the royal physician, Robert Merivel (Robert Downey Jr.), to marry her under the proviso that they never become intimate. Merivel agrees, only to discover he can't resist Celia, and soon finds himself out of the king's favor. Destitute, he devotes himself to helping Londoners suffering from the plague, and in the process falls in love with an equally poor woman, Katherine (Meg Ryan).