Midnight Lace (1960)

In director David Miller's Hitchcock-style suspenseful mystery thriller - (the trailer warned: "Please, don't reveal to your friends the shocking surprise ending!"):

• during the opening, pre-credits sequence, vulnerable but carefree American heiress Katherine "Kit" Preston (Doris Day), newly married over three months to wealthy London businessman-financier Anthony "Tony" Preston (Rex Harrison), strolled through pea-soup thick fog in London's Grosvenor Park Square across from her home after visiting the American Embassy and Consulate General's offices (over a lost passport), and was accosted/stalked by a mysterious, high-pitched, sing-song, puppet-like voice
• an eerie voice made threats upon her life: ("Mrs. Preston, over here! So close, I can reach out and put my hands on your throat....Here, over here now, Mrs. Preston, by the statue of your late President"); when she called out: "Who are you? What do you want?", she was told: ("You'll know when the time comes, Mrs. Preston, just before I kill you"); after she fled and hid behind a park bench, the voice cautioned: ("Careful, Mrs. Preston, I wouldn't want you to get hurt, not yet!"); she ran for home and finally reached the safety of her residence, and took an elevator to an upper floor, while the credits began
• after entering her residence, she received a hug and reassurances from her husband Tony after she told him about being threatened by a "horrible" voice; he theorized that it was a practical joker drawn out by the fog: ("You promise you won't hit me if I tell you something?...Someone's been pulling your leg!"); she stated that the voice uncannily knew her name and that she was an American
• she worried that everyone (her husband, Scotland Yard official Inspector Byrnes (John Williams), and others) doubted her sanity and believed that she was delusional: ("You don't believe me, do you? You think I made him up")
• then, she received a series of anonymous, threatening mysterious phone calls
• the climax included Tony's startling explanation to Kit about how he had planned to throw her off their terrace, to make it look like an "accident" - a suicide due to her presumed insanity, so he could benefit from her inheritance, but then he told her about an "unexpected visitor" or intruder that changed his plan and Tony was forced to fight him off: ("Unfortunately, in trying to defend yourself against a murderer out on the terrace, you fell to the street below. I was upstairs dressing. I ran down when I heard the commotion and tried to stop him. In the scuffle, he accidentally shot himself with his own gun. Even better than my original plan")
• Tony then revealed his motive - he wished to cover up his embezzlement and the indebtedness of his own company: ("Money. I had to find a way to put a million pounds back into my company")
• Tony divulged his major accomplice - their neighbor and his pretty mistress Peggy Thompson (Natasha Parry), who had pushed Kit in front of a bus and made the terrifying phone calls
• he also revealed that the intruding "murderer" was Peggy's cuckolded husband Roy Ash (Anthony Dawson) with a scarred face (who wanted to kill both Peggy and Tony for conducting an affair), but he was seriously wounded when struggling with Tony in the dark apartment and the gun went off
• in the concluding sequence, Kit fled for her life to the scaffolding outside her apartment building, where although fearful of heights, she was able to descend and escape to the street unharmed with the helpful rescue and urging of building contractor Brian Younger (John Gavin)

Tony's Accomplice and Mistress Peggy Thompson (Natasha Parry)

Kit Saved From Harm After Descending the Outside of Apartment Building

• the film's last line was delivered by Tony to Inspector Brynes as he was arrested and surrendered: ("We, as always, underestimate the British. How could I have made the same mistake?")