A friendship develops rather than a romance. As the years pass, they strike out in different directions but always stay in touch. Serious-minded Emma has a flirtation with theatre then gets stuck in a dead-end job in a Tex-Mex restaurant. The more carefree Dexter goes travelling and ends up in television.
He becomes famous and TV's golden boy, before going off the rails. Conscientious Emma grows up and starts buying furniture. Despite their differences, the attraction between them simmers - never quite reaching full boil. Their relationships with others fail but their friendship somehow survives, their lives intersecting time and again as they head towards 40.
I'm against spoilers, so won't give away the ending, but I can say that there's a big jolt coming and I didn't expect it at all.
This is a story that could easily have been trite and corny. But Nicholls has done a brilliant job with it.
He unfolds the narrative at a leisurely pace, letting us get to know Dexter and Emma properly so that, although there's a fair amount of stereotyping going on, they seem like real and relatable people.
All of us have lived through stuff like this: we've had long friendships that have waxed, waned and shifted, have wasted time in the wrong jobs or in the wrong relationships.
Dexter and Emma's story seems real. Moving at times, always entertaining and often funny, it reflects the lives of a whole generation.
And I really liked the format - the regular annual date with the characters - which the author has described as being a little like looking through a photo album. If you've never encountered Nicholls' work before then I'd describe him as very much in the vein of Nick Hornby or Jonathan Tropper - both are known for their emotionally truthful, comic writing.
The movie version of One Day stars Anne Hathaway and Jim Sturgess and has the potential to be a stinker. The trouble is if you lose all the subtleties of the book, the offbeat humour and the sheer humanness of the characters then you'll lose the magic too. And what you're left with is a standard, light romantic comedy.
Having watched the trailer that's certainly what it smells like. So take my advice and read the book first.