On a beautiful, cloudless day, a young couple celebrate their union with a picnic. Joe Rose and his long-term partner Clarissa Mellon are about to open a bottle of wine when a cry interrupts them. A hot air balloon with a 10-year-old boy in the basket and his grandfather being dragged behind it has been ripped from its moorings. Joe immediately joins, along with several other men, in an effort to bring the balloon to safety, but in the rescue attempt, one man, John Logan, dies.

Another of the would-be-rescuers is Jed Parry. Joe and Jed exchange a passing glance, a glance that has devastating consequences and that indelibly burns an obsession into Jed's soul, for Jed suffers from de Clerambault's syndrome, a disorder that causes the sufferer to believe that someone else is in love with him or her. Delusional and dangerous, Jed gradually wreaks havoc in Joe's life, testing the limits of his beloved rationalism, threatening Clarissa's love for him, and driving him to the brink of murder and madness.