As was typical of many of Lewton's horror films, he was given the film's title and, with the aid of credited writers, he created the story around the title, in this case borrowing elements of Charlotte Brontë's Jane Eyre. Also typical of many Lewton films, the supernatural is treated ambiguously, and it's never entirely clear how some events should be interpreted.
Betsy is hired to care for Jessica Holland (Christine Gordon), the wife of Paul Holland (Tom Conway), who is the owner of a sugar plantation on the Caribbean island of Saint Sebastian. She arrives on Saint Sebastian, which is populated by the descendants of African slaves and a small white community of doctors and officials. Betsy is taken to Fort Holland, where she is told the story of how the Hollands brought the slaves to the island, learning that the statue of "Ti-Misery" in the Hollands' courtyard comes from the figurehead of a slave ship — it is an effigy of Saint Sebastian with arrows in it.
That night at dinner, Betsy is joined by Paul’s half-brother Wesley Rand (James Ellison) and told about their mutual mother, Mrs Rand (Edith Barrett). While getting ready for bed, Betsy hears a woman crying from across the courtyard. She goes to investigate and ends up in the tower stairwell. She begins to ascend the stairs when she is cornered by Jessica Holland, who walks towards her like a ghost in a light robe, her eyes staring. Betsy screams for help, waking most of the household. Paul takes Jessica back to bed.
The crying was actually being done by Alma (Theresa Harris), a maid of African descent. Paul explains that the misery of life for the slaves and their descendants means that "they still weep when a child is born—and make merry at a burial."
On a day off, Betsy goes to town and encounters Wesley Rand. While he drinks himself into a stupor, a calypso singer (Sir Lancelot) performs a song about how Jessica had an affair with Wesley but was struck down into a permanent state of shock when Paul would not let them leave. He stops and apologizes to Wesley when made aware of his presence. In the evening Betsy meets Mrs Rand, Wesley and Paul's mother, who is herself a doctor. At that moment the singer can be heard: Her eyes are empty and she cannot talk, And a nurse has come to make her walk. The brothers are lonely and the nurse is young And now you must see that my song is sung. Back at the Fort, as the night draws in, distant drums of a Voodoo ritual underscore an argument between Paul and Wesley during dinner. Paul is trying to cut down on Wesley's drinking but his half-brother accuses him of trying to impress Betsy and of driving Jessica insane in the first place.
Later, Betsy is drawn to the sound of Paul playing the piano. When he sees her approach, Paul apologizes for bringing her to the island and admits that he may have been the cause of his wife's condition. Over time, Betsy has been falling in love with Paul who is a cultured gentleman in spite of his moody personality. She determines to make him happy by curing Jessica.
Betsy administers insulin shock to Jessica with no effect. Alma the maid then tells her of how a Voodoo priest brought a woman out of a similar state of catatonia after the death of her son. Mrs Rand claims that it was just "a little practical psychology", but, ignoring her advice, Betsy takes her patient to the houmfort (a place where the local voodoo worshipers assemble).
The two women set off on a nightmarish journey through billowing fields of cane, past animal sacrifices and to the crossroads guarded by the towering figure of the zombie Carre-Four (Darby Jones) (carrefour is French for crossroad; here it is a reference to the loa Maitre Carrefours). They finally reach the Houmfort where they watch the rituals being performed by a priest known as the Sabreur (Jieno Moxzer), who wields a saber. Visitors are given advice through a hole in a shack by another Voodoo priest. Betsy however is summoned inside where she is shocked to find that the priest is none other than Mrs Rand.
Mrs Rand explains that she uses Voodoo to convince the natives to accept standard medical practices and tells Betsy that Jessica can never be cured. Outside, Jessica's strange demeanor excites the locals who try out a test with the sword. It convinces them that she is a bloodless zombie. Betsy takes her back to the house.
The natives start to intensify their drumming and demand that Jessica be returned to them for "ritual tests". Tension starts to build in the area to the point that it worries even the white community, which has always taken Voodoo as a sort of joke. It's even suggested that Jessica should be sent to an asylum in Saint Thomas in the Virgin Islands. Later the Sabreur sends Carre-Four into the residence but, before he can seize Jessica, Mrs Rand appears and orders him back to the Houmfort.
Paul suggests that Betsy should return to Canada, as he is fearful of demeaning and abusing her as he did Jessica. She is convinced that he is not really like that.
The next day, Doctor Maxwell comes to announce that the native unrest has sparked an official inquiry into Jessica's illness. With obvious glee and satisfaction, Wesley pins the whole thing on Paul, but then Mrs Rand reveals that Jessica is not really ill but an actual zombie, a member of the living dead!
Although she had never actually taken Voodoo seriously before, Mrs Rand claims that when she discovered that Jessica was planning to run away with Wesley and break up her family, she felt herself possessed by a Voodoo god. She then put a curse on Jessica, turning her into a zombie.
Paul, Maxwell and Betsy dismiss all this, claiming that Jessica never actually died and that her heart is still beating — which would rule out her being a zombie. Wesley, on the other hand, gives credence to his mother's story and becomes obsessed with freeing Jessica from her current state of being. He even asks Betsy if she would consider euthanasia but she refuses, claiming that as a nurse she is supposed to guard life, not take it.
Using a plastic doll made to look like Jessica, the Sabreur takes control of her and makes her walk out of the estate after Wesley has opened the gates. Then — perhaps prompted by the Sabreur, although this is never made clear — Wesley pulls an arrow from the statue of Ti-Misery and follows her. As the Sabreur stabs the doll with a pin, Wesley thrusts the arrow into Jessica. He then carries her into the sea as Carre-Four follows, staring blindly into the night.
Later, the natives discover the bodies of Jessica and Wesley floating in the surf and carry them back to Fort Holland, where Paul comforts Betsy.
The original screenplay put more emphasis on Jessica's past as a manipulative hussy. It even had a happy ending with Paul and Betsy back in Ottawa, married and enjoying the hustle and bustle of a modern city. This scene was not included in the final film.
I Walked... was a critical and financial success. It has since been acclaimed for its nuanced performances and haunting atmosphere, particularly the nightmare journey undertaken by Betsy and Jessica in the cane field. It is not a "zombie movie" in the common sense of the term, but it is arguably more accurate and sensitive in depicting Caribbean culture and Voodoo rites and beliefs than many such horror films. Many modern critics are also quick to point out the complex and non-stereotyped way in which black characters are portrayed.
Ambiguity is at the core of the film. It is not established for certain if Jessica is a zombie or not or that Voodoo magic is really effective. Medical explanations are given for Jessica's condition and Betsy gets past Carre-Four even after she has lost the Voodoo patch that she was told she needed in order to do so.