Lady Avril Weyre is told by Doctor Harding that she has a weak heart. He suggests that she get plenty of rest and give up on the extramarital affairs she has been having, especially given her husband's political prospects. Harding informs Sir Julian Weyre of his wife's state, telling him and his secretary Alan that she must take things easy to safeguard her health and that he has written a prescription for her.
Julian later quarrels with Alan, refusing to let him marry his daughter Judy. Alan is sure that this is because his father was imprisoned for treason. Julian placates Alan, but insists that he will not let him become part of the family. During a garden party, Alan exchanges glances with Avril and tells Judy that he still hopes to be able to clear his father's name. Lady Avril is due to visit her family while Julian is away on business and Alan is in Paris. Alan changes his travel plans and meets up with Lady Avril. They travel to the New Forest Inn where they intend to spend the weekend together. Alan make sure that she signs the register on their behalf. The maid, Effie, cheekily makes it clear that she thinks that the surname Avril and Alan are using is false.
Robb, the innkeeper, gets furious with the maid Effie for dancing in the bar and closes it early. Effie clearly likes Alan's company and listens to the radio with him before he goes upstairs to see Avril. She is expecting a romantic encounter, but is furious when he reveals that he wants her to confess what she knows about his father's unjust imprisonment. He threatens to reveal their assignation and hurt Julian's political chances, so she agrees to write a note and explain what really happened. After writing the note, she tries once again to entice him romantically and becomes angry when he rejects her. She throws him out of the room and refuses to hand him the note.
Listening to the radio downstairs, he hears a warning that the prescription that he and Avril collected mistakenly contains poison. He rushes upstairs, but she is dead. He can't find the note and unsuccessfully tries to prise the medicine from her hand. He leaves to catch the boat to Paris.
Effie finds the body and only raises the alarm after finding and reading Avril's note. The doctor tells the Inspector that she died of a heart attack. Julian arrives at the inn and tries to minimise the scandal, while Robb insists that there be a proper inquest so that no stain attach itself to his establishment. Judy collects Alan from the airport and takes him to the inn. Alan confesses to Judy that he was the man who accompanied Avril to the inn. With Effie's help Alan is able to escape without being identified by Robb. The inspector finds Alan's monogrammed handkerchief in the shed where he parked the car the night before.
Julian returns and refuses to speak to Alan during dinner. After receiving a phone call, he tells Alan that the car has been found at Southampton and so it is assumed that the driver fled the country. Julian knows that Alan arrived late in Paris, as Judy tried unsuccessfully to find him at the hotel there. He tricks Alan into a confession by pretending that the medicine Avril had was not the one poisoned and that he has given it to Judy instead. Alan admits they bought it together on the way to the inn.
Julian calls in the staff of the Inn and they identify Alan as the man who registered with Avril. Effie brings out the note she found. In it Avril admits that she was guilty and that Alan's father was innocent. Julian tells Judy and Alan that he found this out after it was too late and was afraid to admit it as it would affect his political career. Alan gives him the letter and then stands beside Judy.
