Plot

Christine Clay (Pamela Carme), a famous actress, receives a surprise visit from Guy (George Curzon), a husband she hasn't seen in eight years. Guy reminds her he "dragged her out of the gutter to make her a star". He's furious that she left him, outraged over her "silly Reno divorce" and indignant that she is playing around with young men, in particular a struggling writer staying near them at their retreat on the English coast. They argue vehemently until a furious Christine slaps her husband's face four times. He keeps calm except for his eyes, which twitch uncontrollably, and departs.

A day or two later, Robert Tisdall (Derrick De Marney), a local writer, happens to be walking along the seaside cliffs when Christine's body washes ashore and he sees her body on the beach. He runs to get help or call the police and is seen by two young female swimmers who mistakenly think that he is the escaping murderer.

Police arrive and arrest Tisdall on the testimony of the two witnesses. Also, he is known to own a raincoat matching the murder weapon, a belt coiled in the form of a large question mark that lay near the corpse. Robert claims innocence, but in the hostile crowd that has gathered, none believe him. At first Tisdall is confident that it's all a misunderstanding.

After an overnight interrogation at the police station, it's established that he knew Christine and is struggling economically, so his motive seems to be £1,200 left to him in her will. Robert faints, and is revived by Erica Burgoyne (Nova Pilbeam) the attractive young daughter of the Chief Constable, Colonel Burgoyne (Percy Marmont). She seems quite at ease in ordinary male company, talking back vigorously to make her points or get her way.

Saddled with a hopelessly incompetent and greedy barrister named Briggs (J.H. Roberts), Tisdall believes his innocence will never be proven, as the claim that his raincoat was stolen at a lorry station a few days ago is not believed. As he is about to go on trial, he escapes from the crowded courthouse corridor by donning his defense lawyer's glasses, then stows away in Erica's dilapidated Morris automobile. A reporter with a camera (Alfred Hitchcock) is at the steps of the court house and spots Tisdall escaping, but takes no notice.

Robert explains his situation to Erica and vows to recover the lost raincoat, identify the real killer and prove his innocence. At first, Erica doesn't believe Tisdall is innocent, but she is attracted to the young man, so she continues to help him elude capture and search for the stolen coat that might lead to the real killer. She drops him off at an abandoned mill so he can hide out.

At dinner at home, Erica's brothers discuss the suspect's escape, his need for money, sleep, and food to stay on the run.

The next day she brings him food, a bit of money, and they have a talk in depth which changes her attitude towards him. "I can laugh because I'm innocent," he tells Erica. "You don't believe me. I wish you did." After eating, Robert nonchalantly flips a wrapper through a broken window at the precise moment that local foot policemen happen to glance in that direction. Two policemen come into the mill, and Erica and Robert must rush off in the Morris. They are spotted together, so the police suspect she is his accomplice.

During their flight, they stop at the home of Erica's aunt, where her seven-year-old daughter Felicity is having a birthday party. Erica enters alone, planning to just call her father and check in, but she is dragged into the party. Uncle Basil (Basil Radford) arrives and finds Robert waiting outside and brings him inside, assuming he is Erica's beau. Aunt Margaret (Mary Clare) becomes suspicious. Just as they get away in the Morris, Margaret calls her brother Chief Constable Burgoyne, who in turn alerts police to be on the lookout for them.

Their search takes them to "Tom's Hat", the diner where the raincoat was stolen, where they learn that a china mender known as Old Will got a new raincoat recently, and that he hangs out at a lodging named Nobby's. Erica's attitude ends up triggering a brawl and Robert needs to rescue her.

They find Nobby's, Robert changes into seedy clothes, enters and asks about Old Will, after leaving Erica to sleep in the car hidden behind the freight train, as she can't stay awake any more.

The next morning, in the lodging house full of snoring vagrants, dead cockroaches, and peeling paint, Tisdall finally meets the colorful hobo named Old Will (Edward Rigby). After hearing the story retold, Will agrees to help them find the man who gave him the garment, a stranger with noticeably twitching eyes who presumably is the murderer.

Erica's dog, meantime, has smelled that Will is actually wearing the raincoat in question, three layers down from the outer one.

Police cars come chasing, and they succeed in losing them by crossing railroad tracks in the nick of time before two long trains block the way, and a huge trailer truck further delays them at the gate beyond.

They attempt to hide by driving into an abandoned mine, where the Morris starts to sink into tunnels below, and Erica has to be pulled to safety by one hand by Robert in the nick of time. The police find the mine, Robert flees but Erica is caught. She is brought to her father who is about to tender his resignation over the incident.

That night, after she has a serious talk with her father, who shows her a letter of resignation, she is in her bedroom when Robert comes in the window, and she rushes to hug him. He tells her lovingly that he will turn himself in, shortly, to spare her anguish and relieve her conscience. As they talk, he learns that in a pocket of the coat, matches from the Grand Hotel have been found, a place where Tisdall has never been.

Together they decide that's a last clue worth pursuing. Erica will take Will there to see if the twitchy eyed man may still be around, and Robert will find his own way there independently.

Old Will dresses up in a new uncomfortable morning coat in order to infiltrate the hotel with Erica, but spotting a man with a twitch in the large crowd proves impossible.

In a memorable sequence, the camera moves forward through the hotel lobby to the ballroom and the dancers to finally focus on the drummer in a dance band performing in black-face.... and his eyes twitch.

The drummer, Guy, recognizes Old Will in the audience, and seeing the policemen, unaware that they are there to arrest Tisdall, the man performs poorly due to increasing nervousness and fear, to the point he is berated by the musical conductor. Guy proceeds to have a breakdown, precipitated by a drug he takes to excess to try to control the twitching. He begins banging drums and cymbals out of syn-ch, helplessly falling over the instruments, and passes out while the other musicians stare at him.

Just then, the police have found Robert and Old Will and Erica, and are escorting them out. As hotel staff ask around to find medical help for Guy, Erica offers her experience, and the police, who know her, allow her to approach the unconscious Guy. As Erica tends to him, his eyes twitch, she realizes who he might be. Immediately after being revived, Guy confesses his crime and laughs hysterically. The police drag him away, cackling maniacally.

In the final scene, Robert Tisdall and Erica Burgoyne are united. Her father arrives and seems happy to shake his hand, as if welcoming him into the family.

THE END