Joe and Marie are having a child. During a scan, the doctor tells Marie that she should take things very easy. She gives them a photo of the scan. Later that evening, Joe goes to see Beth, his mistress. He looks at the photo she took of him on a boat in the loch, which is strangely covered in a blue fog. She jokes that maybe the 'Blue Boy' is going to get him. He insists that they end their affair.

Joe and Marie open a letter that arrived just as they were preparing to leave. It's Beth's photo, and on the back she has written, 'I'm sorry for you'. Joe reassures Marie that everything is all right. While driving to the hotel, they hit a deer with the car. When they arrive, the manager Robert mistakes Marie for Beth. That night, Joe awakes from a dream in which he is observed by a young boy while having sex with Beth in the hotel room. Marie is also awake, convinced she can hear distant crying in the wind.

At breakfast, the waitress, Christine, gives Joe a phone number to call. She tells Marie that the 'Blue Boy' spoils all photos taken on the loch. Joe calls Beth back, but she hangs up. Marie wants Joe to guess the sex of the child by hanging a ring over her stomach, but he refuses. At the worksite, Marie answers a phone call from Beth and takes a message. Just as the photo of the baby scan blows off the bed, there is an accident with the logging machinery and Joe is lucky not to be injured. Marie is made suspicious by the phone calls and rings the number Beth left earlier, saying on the answer machine that she fells sorry for her.

Marie asks Robert about the fountain in front of the hotel. He tells her that it is in honour of a child who drowned in the loch who may be related to Christine. He warns Marie not to take what she says too seriously. Marie goes to the church where Christine plays the organ. She notes that no baby has been placed in the manger as part of the Christmas display. Christine tells Marie that she is the last of her family and the boy was a distant relative. He was recovering from typhoid but prone to sleepwalking and fell into the loch while looking for his mother. By the time he was found, his body had turned blue from the cold. Marie nearly faints, thinking of her previous miscarriage.

Marie goes to church the next day and sees that the baby is now in the manger. Christine thinks that Marie is sensitive to the past. Little wet footsteps were found going back to the hotel on the day of the boy's death. She thinks his spirit is still there and that he needs Marie's help. Marie is frightened and asks Christine to stop telling her these stories.

Joe and Marie fight about his affair and he leaves, but just misses the ferry. He passes the night in his car. Christine tells Marie that the boy stayed in her room. He was looking for his mother then, and has now found Marie. When Joe returns, Marie is furious with him and the two hit each other. He tells her that he loves her and wants them to stay together. Robert tells Marie that the father of the 'Blue Boy' was actually having an affair with the child's nurse while the mother was in Glasgow.

Joe and Marie realise that they have both become unsettled by the story of the 'Blue Boy' and decide to leave. While packing, Beth arrives. Joe drives her to the ferry, insisting that he will never leave Marie. On his journey back, he sees the face of the 'Blue Boy' in his rear view mirror. To avoid hitting Marie, who has come out to meet him, he swerves and crashes into the loch. The photo of the scan is juxtaposed with the face of the 'Blue Boy'. Marie wades in to the water, to no avail. Months later, she looks at her baby in its cot.