The novel traces the relationship between Emma Morley and Dexter Mayhew over two decades, revisiting them each year on the same date - July 15th (St. Swithin's Day). The novel begins on July 15, 1988, when Emma and Dexter graduate from university and spend the night together, though they do not become a couple. Each subsequent chapter takes place on the following July 15th, chronicling the evolution of their friendship, careers, and eventual romantic involvement despite their contrasting personalities and life paths.

Emma dreams of an idealistic life while Dexter seeks fame and success. Over the years, Emma struggles with unfulfilling jobs while Dexter becomes a wealthy TV personality with substance abuse issues. Their connection remains strong through letters and reunions, though circumstances continually drive them apart.

Ultimately, Emma and Dexter realize their love for each other, but their romantic relationship is tragically cut short when Emma unexpectedly dies, leaving Dexter devastated.