Following the end of World War Two, Mary Justin flies to Switzerland for a holiday on Lake Annecy. It is her first foreign visit and she expects her husband Howard to join her later. Steven, an old flame, is staying in the adjoining hotel room. Mary recognizes his voice on the balcony, although they haven't met for nine years.

Mary recalls meeting Steven at a New Year's Eve Chelsea Arts Ball before the war, after several years without seeing him, during which time she had married Howard. Steven is there with his girlfriend Pat. Years earlier, Mary and Steven were lovers and he asked her to marry him. Valuing her independence too much, she refused him. Steven bitterly told her that her life would be a failure.

Following the Ball, Howard goes away on a business trip, and Mary finds herself thinking about Steven, who is now a biology lecturer. They arrange to meet at his flat and he cooks lunch for her. Before long, they have resumed their old affair.

Howard returns from his trip. Mary tells him that she and Stephen are going to the theatre to see the play First Love, but they go out for dinner and dancing instead. Mary agrees to tell Howard about her love for Steven. Howard realises that she has left the theatre tickets at home and becomes suspicious - and jealous, so much so that he goes to the theatre himself, and sees their empty seats.

When Steven takes Mary home, Howard asks him in for a drink and asks them about the show. Mary realises that Howard knows they didn't go to the theatre, and they confess their love. Howard grows angry and Steven leaves.

Steven receives a letter from Mary, in which she says they must part, but he suspects that Howard made her write it and goes to the house to confront him. Howard says that by marrying him, Mary turned her back on romantic love with Steven. Just then Mary comes in and tells Steven that she will stay with Howard; she needs the security he can give more than the grand passion Steven can offer her.

Nine years later. Mary has run into Steven again, at her holiday hotel. He is now a professor. They meet and spend the day together in the mountains. Steven tells her that he married Pat and now has a family, with whom he is very happy. However, childless Mary stills seems to be in love with him. Howard arrives at the hotel. From the terrace he sees Mary and Steven returning together in a motorboat. Steven is about to take the same boat to the railway station and they part, but Howard sees their farewell kiss and misinterprets the situation. Back in London he files for divorce.

Mary desperately tries to locate Steven, who is travelling through Europe. She finally catches up with him at Victoria station - where his wife is waiting for him. He is served with the divorce petition before Mary can warn him.

Mary tries to stop the divorce proceedings. She meets Steven and tells him that her lawyer has told her there will be no case, but she is lying. She returns to the house which she no longer shares with Howard. He returns from another business trip and she asks him to stop the divorce but he turns on her angrily. He then realises that he really does love her, but she has already left before he can tell her. He follows her to the nearby Tube station and arrives just in time to stop her from jumping in front of a train. He takes her home.