In 1840's London, esteemed surgeon Dr. Thomas Bolton (Boris Karloff), who is renowned and respected by his peers for both his speed and his efficiency, is trying to find a way to perform painless surgeries. You see, in those days before anesthesia, anyone needing an operation or limb removed were simply strapped down to a table and forced to undergo the painful procedure completely conscious, which often led to some nasty psychological complications later on. At his home-lab, Dr. Bolton begins experimenting with nitrus oxide and other chemicals he thinks may alleviate pain, using himself as a guinea pig and keeping notes of all his findings in a small notebook. When he thinks he's made a breakthrough, he presents his findings to the medical board and is humiliated when things backfire and his patient freaks out. He returns to his lab and decides to begin experimenting with opium. Unfortunately, he's already created too much skepticism from his peers, who've put a block on his access to chemicals and refuse to let him do another demonstration on his findings. Even worse, after dozens of times inhaling the opium compound, he's become a pathetic drug addict who's quickly losing his surgical skills and suffers from blackouts and periods of amnesia.
Whilst on the chemical, Dr. Bolton stumbles into a rowdy tavern in the grotty Seven Dials district and gets hopelessly immersed in the schemes of several shady characters who have been selling suspiciously fresh corpses to the same medical college Bolton's son Jonathan (Francis Matthews) attends. Burly tavern owner Black Ben (Francis De Wolff), with help from his lover Rachel (Adrienne Corri) and a facially-scarred thug named Resurrection Joe (Christopher Lee) have been killing the drunken and destitute who wander into the establishment. They get their hands on Dr. Bolton's medical notes and blackmail him into signing the death certificates of their victims. Bolton eventually finds himself hiding out there in a back room, carrying on with his experiments and slowly succumbing to both his addiction and delirium, while the authorities (led by Nigel Green) start closing in.
Despite the fact this wasn't released until four years after it was made and often on a double bill with the fun if inferior Werewolf in a Girl's Dormitory (1961), this is a very well made period horror-drama with top notch sets, a strong screenplay and excellent performances. It's an especially good showcase for Karloff, whose well-intentioned character always manages to engage our sympathies even when he's hopelessly siding with evil. Watching the actor go from a driven and caring doctor to a weakened, drug-addled, mindless shadow of his former self is one of the major reasons to see the film, but there's also good work from De Wolff, Corri and much of the rest of the cast. In support, Lee; tall, lean, strikingly dressed in a tophat and all black attire and often standing silently in the background, makes the most of his role as a remorseless killer. The cast also includes Betta St. John as Karloff's daughter-in-law, Finlay Currie as a colleague, Basil Dignam as the hospital board chairman and a young Yvonne "Warren"/ Romain as an abused barmaid.
Day also made the Karloff vehicle The Haunted Strangler (1958) for the same company (Amalgamated Productions). Criterion picked the film up for distribution and have put out an excellent quality DVD with plenty of extras. They also released it as part of a "Monsters and Madmen" box set, which includes the two Karloff/Day collaborations as well as Day's First Man Into Space (1959) and the sci-fi / thriller The Atomic Submarine (1959).