It is set in 1761 at St. Mary of Bethlehem hospital, the oldest continually operating mental institution in the world, more popularly known as "Bedlam." Karloff plays George Sims, the master of Bedlam. Sims is a cruel and vicious man who beats and starves his inmates and locks them in cages. He also treats them as zoo animals, putting them on exhibit for anyone who forks over two pence.
One of the people who pays tuppence to see the inmates of Bedlam is Nell Bowen (Anna Lee), an actress and paid companion of Lord Mortimer, a patron of Bedlam House. Nell is shocked by the abuse suffered by the inmates of Bedlam, and repulsed by the oily Sims, who sucks up to Lord Mortimer and arranges for his mental patients to put on a show for Mortimer's amusement. She resolves to reform Bedlam, which makes an enemy of Sims. When she insults Lord Mortimer for his chumminess with Sims, she makes an enemy of him as well, and the two of them conspire to have Nell committed to Bedlam as a patient.
Third and last collaboration between Karloff and Lewton. Not to be confused with television series Bedlam.
- Aluminum Christmas Trees: Latter-day viewers might not understand when Nell says to Hannay "Are we lovers that you 'thee and thou' me"—viewers in 1946 might not have understood for that matter. But it is true that when "thou" was used in English it was the familiar, intimate form of address, the way one would talk to a lover or a husband or a child.
- Aristocrats Are Evil: Lord Mortimer isn't as overtly, actively evil as Sims, but he really is quite the bastard nevertheless. He chortles and guffaws like the fat pig he is when the inmates of Bedlam make a show for his entertainment, and he doesn't bat an eye when one of the mental patients collapses and dies from the golden paint all over his body.
- Bedlam House: Surprised? The Real Life Bedlam is the Trope Namer and was every bit the hellhole that it's portrayed in this movie.
- "Blackmail" Is Such an Ugly Word: Bribery instead of blackmail, and inverted in any case. But when Hannay the mason asks for a construction job, and Sims demands a two-guinea kickback in return for giving him the job, Hannay says a lot of words to the effect of I'll Pretend I Didn't Hear That. An indignant Sims shoots back with "What kind of cant is this? I've asked you for a bribe, man!"
- Body Paint: Nearly 20 years before Goldfinger embedded an Urban Legend in the public consciousness, one of Sims's inmates dies from the full body-paint job he got to play a role in Sims's little pageant. The nonsense about breathing through the skin is even discussed.
- Buried Alive: After the catatonic woman stabs Sims In the Back with the trowel, the inmates panic. They resolve to hide the body by walling it up, and to make out that Sims ran away in order for them to avoid blame. They proceed to do so—only for Sims to open his eyes and reveal that he is still alive right before the last brick is put in place.
- Call-Back: When Hannay first meets Nell and talks to her as Quakers did, she says "Are we lovers that you 'thee and thou' me?" In the last scene she starts addressing him by "thee" and "thou", signaling that she has accepted him as a lover, and he wryly repeats this line back to her.
- Call-Forward: One of the inmates demonstrates a page-flip cartoon he's made of Nell ministering to the other inmates, and remarks that "If I could only get a light behind these pages I could throw them large as life up on the wall." The person he's addressing says they could charge admission.
- Catch-22 Dilemma: How Sims keeps Hannay from visiting Nell in the asylum. There's a rule stating that all visitors must deposit their weapons in the front before taking the tour of the asylum. Hannay can't deposit his weapon because he doesn't have one; he's a Quaker. So since he won't deposit a weapon, Sims won't let him in.
- Chiaroscuro: Used several times to make the asylum's interiors more creepy, most dramatically in the scene where Hannay has snuck in and has to make his way through the detention wing for the violently insane, who reach out and grab at him from their cages as he passes.
- Come to Gawk: For two pence, Sims lets visitors come in and gawk at the inmates of Bedlam. This really happened at the real Bedlam in this era.
- Compensated Dating: Essentially what's going on with Nell and Lord Mortimer. She makes pains to let people know that she is not The Mistress. In fact she is a paid companion whose job is to amuse Lord Mortimer and make him laugh.
- Corrupt Bureaucrat: Sims maneuvers his way into getting people he doesn't like chucked into his asylum. He also takes kickbacks from contractors who work at Bedlam.
- Dude, She's Like, in a Coma!: When characterizing his various inmates as "dogs" who need beating, or "tigers" who need to be locked in cages, Sims brings Nell to a very young attractive who is standing against a support beam, staring out in a catatonic trance. He says "Some, like this one, are doves," and creepily caresses her cheek. It's strongly implied that he's sexually abusing her. Notably, at the end it's the catatonic woman who stabs Sims In the Back.
- Eye Open: The horrifying moment in which Sims's eyes fly open to reveal he's still alive, right before he's Buried Alive by the last brick in the wall.
- Getting Crap Past the Radar: When Nell is introducing herself to John Wilkes, he makes clear that he expects more than Compensated Dating with his companions; if she lives with him she'll have to put out. He isn't gross about it, though, and they shake hands and part as friends.
- Historical-Domain Character: Nell meets John Wilkes, who in the movie as in Real Life is a liberal reformer.
- Kangaroo Court: The ridiculous farce of a sanity hearing by which Sims gets Nell clapped into Bedlam. Sims is whispering in the ear of the head examiner throughout.
- Lady Drunk: Sims's niece, who is brought to Sims by Mortimer as a much more low-rent replacement for Nell. She insists on drinking gin instead of being more genteel and drinking wine as Sims tells her. She's also rather bitter and sarcastic.
- Ye Olde Butcherede Englishe: Practiced In-Universe by Hannay the Quaker, who not only uses "thee" but uses it all the time, including when he should say "thou." This is how Quakers of the day really talked