Couples adorned in costumes reminiscent of the 1900s whirl around a room, elegantly swaying to the melodic strains of The Merry Widow waltz by Franz Lehár. Amidst this festive atmosphere lies a man, deep in contemplation, dressed in a suit, sprawled across a bed. Bills of money clutter both his dresser and the floor nearby. The landlady, Mrs. Martin, knocks and enters the room, informing “Mr. Spencer” (Joseph Cotten) that two men are inquiring about him. Following his previous instructions, Mrs. Martin claims to have told them he is not available, though they remain agitatedly waiting outside at the corner.

Spencer, whose real identity is revealed to be Oakley, rises from the bed, gathers the scattered money, and stealthily exits past the two men, who offer no reaction but begin to shadow him at a distance. He moves hastily, navigating corners until he successfully retreats atop a building, observing them from a safe vantage point to ensure he has evaded their pursuit. The scene transitions to a payphone, where Oakley sends a telegram to his sister, informing her of his impending visit to Santa Rosa, California, signing off as Uncle Charlie.

Meanwhile, in Santa Rosa, a teenager named Charlotte “Charlie” Newton (Teresa Wright) lies frustrated in bed, lamenting to her father, Joseph “Joe” Newton, the mundane existence that envelops her family life. Yearning for excitement and adventure, Charlotte resolves to invite her beloved Uncle Charlie, feeling a deep familial bond with him that transcends ordinary understanding. To her delight, at the telegraph office she discovers that he has indeed communicated his arrival, all the while humming a cheerful melody that seems to uplift her spirits.

The rest of the Newton family anticipates Uncle Charlie’s visit with giddiness. Charlotte’s mother, Emma “Emmy” Oakley-Norton, radiates joy at this prospect, even echoing her daughter’s humming. As the train approaches, Oakley takes meticulous care to remain hidden behind the Pullman curtains, presenting himself as a frail old man upon disembarking. His ruse is reinforced when the train clerk (Clarence Muse) and a concerned couple (Edward Fielding and Sarah Edwards) offer assistance, only to receive gruff rejections from him. Once his family arrives, however, his demeanor shifts entirely, as he greets them with boisterous warmth.

The reunion is filled with joy and laughter, punctuated by the presents Oakley brings, including a special ring for Charlotte featuring a prominent emerald. Young Charlie’s youthful curiosity shines through when she comments on how similar she and her Uncle Charlie are, noting his penchant for secrecy. When she questions him about some engraved initials found in the ring, Oakley dismissively attributes it to a mistake made by the jeweler.

During dinner, the air is thick with mystery as Charlotte repeatedly hums her tune, trying to draw recognition from her family. When Oakley suggests it might be The Blue Danube, Charlotte barely masks her excitement as she prepares to declare The Merry Widow. However, Oakley’s clumsiness leads to a spill, causing the family to share a laugh amidst the tension building between Charlotte’s subconscious suspicions and her uncle’s charm.

Unbeknownst to the Newton family, discussions between two men – Herbert “Herbie” Hawkins (Hume Cronyn) and Joe – center around hypothetical scenarios of murder and evasion. Oakley, ever the cunning schemer, turns to the local newspaper and identifies a story that particularly resonates with him. He begins teaching the younger children, Ann (Edna May Wonacott) and Roger (Charles Bates), to fold a newspaper into a house, cleverly tearing out pages for doors and windows while concealing the remnants in his coat pocket.

However, as the days turn into unnerving interactions, it becomes clear that Uncle Charlie is harboring dark secrets. Worry escalates as Charlotte discovers newspapers hinting at her uncle’s involvement with a serial killer, known as the “Merry Widow Murderer”, who seduces wealthy widows only to dispose of them for their money. The tension mounts as Jack Graham (Macdonald Carey), a detective posing as a visitor, reveals chilling truths about her uncle’s past.

Charlotte grapples with the burgeoning realization that her cherished and idolized Uncle Charlie could possibly be implicated in unspeakable crimes. The intelligent strikes of sympathy begin to break down as dinner conversations hint at Oakley’s disdain for riches, and Charlotte’s once-safe admiration begins to crumble.

The story spirals into a thrilling crescendo as attempts on her life seem increasingly targeted. A narrow escape in a garage reveals the true villainy lurking within Oakley as familial affection clashes with sinister motivations. Charlotte’s frights evolve into an audacious confrontation, leading to a climactic struggle aboard a departing train, where his desire to eliminate the last living evidence of his misdeeds culminates in a desperate act.

The community mourns Uncle Charlie’s death, as many breathe a sigh of relief without knowing the dark undertones that made their lives dizzyingly intense. Jack finds solace with Charlotte, who grapples with the burden of her silence; their bond strengthens amidst the backdrop of a chilling family secret, leaving them resolved to keep Uncle Charlie’s gruesome truths concealed from the world.