Maggie Walsh (Ross) and her boyfriend Pete Danner (Elliott) are interior decorators from Los Angeles, California. They are called to England by a potential client, sight unseen. At first they hesitate since both are booked to do work for somebody else in Los Angeles, beginning a few days from now. Suddenly they get a phone call that their local customer has had a fatal accident, which seems awfully coincidental to Pete. Nonetheless, he tags along with Maggie to work for their latest client, who has prepaid Maggie's airfare.
Arriving in England, Maggie and Pete get into a country road accident, one that involves their motorcycle and a vintage limousine belonging to Jason Mountolive (Standing): a moribund and reclusive multi-millionaire (who later turns out to be a second-generation Satanist as well). Apologizing for their unfortunate mishap, this independently wealthy Englishman invites the twosome to stay at Ravenhurst: his rambling country estate. But this is no setback; it was Jason himself who arranged Maggie's trip here, ostensibly to do interior decorations for Ravenhurst. (As Maggie will ultimately discover, it was also Jason who arranged the previous mishap which killed her Los Angeles customer.)
Once installed at Ravenhurst, Maggie and Pete get acquainted with Jason's five (eventual) beneficiaries, including "...a million-dollar prostitute, a star-maker, a nation-killer, a woman whose lust runs cold as graveyard snow..." In other words, various European luminaries who are "beholden" to Jason for helping them with their careers... and with occasional run-ins, public scandals, et al. All five have been summoned to Ravenhurst in a fashion much like Maggie's, because Jason is about to die; in fact, he is wasting away upstairs as they speak. Maggie is astonished to hear this; Jason seemed vigorous, almost youthful, when she and Pete first met him. (It is never explained, in the movie, why Jason's appearance and his health change as greatly and as quickly as they do.)
Jason receives visits from those who have come to see him; indeed, their benefactor lies dying in a sterilized chrome-and-glass-styled bedroom which is equipped with various life-support machines. He calls Maggie to his bed, which is shrouded by white curtains so she cannot see Jason's face. Then his hand, which suggests that of a monster, abruptly reaches out and puts a ring on her finger, a ring emblazoned with the Mountolive family crest. After Jason dismisses her, Maggie rejoins the other five guests, who all wear like rings. She tries to remove the ring, but it has grafted itself to her finger.
Shortly thereafter, the European guests proceed to die via increasingly mysterious and gruesome means. Maria, despite being an excellent swimmer, somehow becomes trapped under the surface of Jason's indoor pool and is drowned. Recording-studio executive Clive Jackson (Daltrey) gets a chicken bone lodged in his throat during dinner and perishes during a botched tracheotomy, although he was having pate but no chicken. Karl Liebnecht (Gray), a German military-surplus dealer, is cremated by a massive tongue of flame which spews dragonlike from Ravenhurst's fireplace yet leaves the rest of the room he's in untouched. A mirror in Barbara's bedroom explodes, impaling her with glass shards like a pincushion, then reforms itself without a crack. Jacques, a hotelier, tries to kill Maggie and Pete because he suspects them of engineering these inexplicable deaths. Just as he gets the drop on them, his shotgun jams and backfires into his face, blowing him off Ravenhurst's roof.
Amid these bizarre tragedies, Maggie and Pete discover - via newspaper clippings - that their fellow guests were each accused of murder, public-property destruction, or worse... yet never paid for their respective crimes. Maggie also discovers that Jason's mother was Lady Margaret Walsingham, while his father was the Lord of Mountolive; both their families were heavily into black magic and witchcraft. Lady Margaret and her husband were burned at the stake for heresy, although their son survived. Maggie realizes that she is Jason's great-granddaughter, and has been lured to England accordingly, albeit under false pretenses.
Maggie is now the last remaining "seal-bearer," the heir to Jason's "Legacy" (hence the title): ostensibly the Walsingham-Mountolive financial empire, which the other five (late) beneficiaries ran for him; but also the sinister powers which he used to sacrifice them to the Devil. She returns upstairs to Jason's room. With his strength and breath running out, Jason confirms that he was responsible for killing all the other guests. He prepares to pass his Satanic abilities and knowledge on to her, with instructions to choose six heirs of her own; when the proper time comes, Maggie will bring about the deaths of all except one who will inherit the powers of Satan.
Pete forces his way into the room and attempts to halt said ceremony by short-circuiting Jason's life-support system. But Maggie and the Legacy have already claimed each other; she is now Lady Margaret, with the entire household staff of Ravenhurst at her beck and call. (The staff includes Nurse Adams, who can transform into a white cat and back.)
Maggie selects Pete as her first beneficiary, or "seal-bearer." She gives him a Ravenhurst crest-ring which, sure enough, grafts itself to his finger and the cycle begins anew.