I, Monster

I, Monster is a British horror film based on Robert Louis Stevenson's story of a doctor who splits his own personality. Dr. Charles Marlowe, a scientist in late nineteenth-century London, develops an intravenous drug intended to free the instincts people keep buried. He tests it on patients first, and the results are disturbing: their hidden selves surface in ways they cannot control. Marlowe then takes the drug himself, and a second personality emerges, the callous and increasingly dangerous Mr. Edward Blake. Each use of the drug makes Blake harder to contain, and Marlowe's colleague Frederick Utterson grows alarmed as the man he knows gives way to something far worse.

Christopher Lee plays both Marlowe and Blake, and Peter Cushing appears as Utterson, the friend and fellow doctor who watches the change. Amicus Productions, the British studio behind the film, had already paired the two actors in earlier horror pictures, and here it adapts familiar material under changed names. The screenplay is by Milton Subotsky, who produced with Max Rosenberg, and Stephen Weeks directed; his next feature as director followed more than a decade later, Sword of the Valiant.

The film runs seventy minutes and was released in November 1971. On IMDb it holds 5.7 out of 10 from around 2,200 votes, and on TMDB 5.5 out of 10 from a much smaller number of votes. Both suggest a modest, mixed response rather than enthusiastic praise, consistent with a low-budget studio genre piece from that period.