The mother must have seen this film and been moved by it. It was released in 1999. The accuracy of her quotation indicates that she had made a considerable effort with these lines. She either replayed it endlessly on video or DVD while she copied them out, or she watched the film so many times that she knew it by heart. Either way, it was strangely moving that this somewhat far-fetched love story had made such an impact on a lonely woman.

Or could the whole thing be a hoax? No, I think that a practical joker would have left some method by which he or she could have measured the success of the joke, in other words a return address. And more crucially than that, I think the emotions are too raw and sincere, and the subject too bleak, for a prankster.

I wondered if she had also seen Castaway, in which Tom Hanks’s character, Chuck Nolan (No-land), gradually discovers that ‘you never know what the tide will bring’. Or another Costner film, Waterworld, set ‘within a vast sphere, ever drifting in uncertainty, driven from end to end’. Given her familiarity with Costner’s later film, it is quite likely. I asked myself whether the woman brooded over her son’s death until this film inspired her to write her letter and consign it to the waves? Or did the idea hit her immediately as she watched the film?

Message in a Bottle does not have a conventional happy ending. Like the letter-writer’s story, it ends in grief but also in a certain acceptance of death, and a forgiveness of earlier errors. The heroine is alone on a beach at the beginning, and alone once again on a beach at the end, but in between she has experienced great love. The cliché ‘Better to have loved and lost than never to have loved at all’ is vindicated and she walks off into the sunset, alone but no longer lonely.

I wept in spite of myself, not so much because I was caught up in Theresa’s experience, but at the thought of the unknown mother’s grief and empathy as she watched this. When I first heard those lines from the letter in the film, I was appalled that the poetic figure of my imagination, the woman who had written so lyrically and heart-rendingly of her loss, had suddenly metamorphosed into a fan of Hollywood slush. Or that it might mean the whole letter was a figment of someone’s overblown sense of drama or humour. But then I realized that I still believed in the letter’s authenticity, I was still convinced that it represented a real tragedy, and that the letter was not tarnished by her plagiarism. Rather, it emphasized our common cultural background. She may be a Frenchwoman with a poetic turn of phrase and a tragic past, but she still watched romantic Hollywood films like the rest of us.

‘The attractions, fascinations there are in sea and shore! How one dwells on their simplicity, even vacuity! What is it in us, arous’d by those indirections and directions? That spread of waves and gray-white beach, salt, monotonous, senseless – such an entire absence of art, books, talk, elegance – so indescribably comforting, even this winter day – grim, yet so delicate-looking, so spiritual – striking emotional, impalpable depths, subtler than all the paintings, music, I have ever read, seen, heard. (Yet let me be fair, perhaps it is because I have read those poems and heard that music.)’