What makes for a modern love story? In an era loaded with cynicism and highlighted by big gestures, newly written books about romances tend to be either post-modern or nostalgically optimistic. Do the characters have to be perfect? Does it have to end well? What does “ending well” even mean now?

These are things that would occur to anyone who read David Nicholls’ “One Day,” an enthralling if bittersweet portrait of a relationship each on the same day over several years. Not only was that 2009 novel a worldwide bestseller, but it was also adapted into a 2011 feature film starring Anne Hathaway and Jim Sturgess.

Nicholls’ characters were attractive but otherwise quite ordinary, going through a relationship as it changes. It is neither saccharine nor nihilistic, but bittersweet to the point of heartburn.

Nicholls brings this same dynamic to his fourth novel, “Us” (HarperCollins, New York, 2014, 392 pages), but in an even heavier concentration. The characters are far more ordinary and authentic; what they go through seems real but also dramatic. Fiftysomething British biochemist Douglas Petersen’s comfortable home life is shaken up when his wife of over 20 years announces one night that she’s thinking of leaving.

“I think our marriage has run its course,” Connie tells him. “I think I want to leave you.”

Additionally, Douglas’ 18-year-old son Albie—with whom he has always had difficulty relating—is also about to leave home. Douglas and Connie had planned a grand trip through the great cities of Europe with Albie so they put off the decision to stay together or not until after the trip.

This is, of course, a terrible idea as Douglas is constantly tormented by the possibility of his marriage ending. He then begins to remember the details of their love story from the beginning.

Here is the Nicholls trick in “Us”: As readers witness the Petersens’ marriage fray and fade, the book contrasts that with the swell of their fresh romance at the start. We watch as they fall in love, as they are seemingly falling out of it, chapter after chapter. Nicholls then adds different elements as they move from one city to another, adding something of an art travelogue feel to the unusual love story.

As readers witness the two opposing forces in the Petersens’ life together, they also get a reflective, devoted portrait of Douglas’ very wary relationship with Albie. It’s more than just a generational conflict; it’s one born of the elemental miscommunication and misunderstanding between a father and a son.

Irresistible

As can be expected, the Petersens’ European sojourn does not go as expected. But Nicholls writes about love really well, and his description of the time Douglas and Connie fall for each other is just irresistible: “Light travels differently in a room that contains another person; it reflects and refracts so that even when she was silent or sleeping I knew that she was there. I loved the evidence of her past presence, and the promise of her return, the way she changed the smell of that gloomy little flat… I don’t mean to strike an inappropriate note, but few things have ever made me happier in my life than the sight of Connie’s underwear drying on my radiator.”

Douglas is a fascinating narrator because he is such a sad sack as well a hopeless romantic. You want to pull for him despite—or maybe because—he has the cards stacked so heavily against him. Connie is perfect because you will both fall for her and hate her, just like Douglas. And Albie may be the most accurate depiction of a disaffected, distant millennial child in recent memory.

Nicholls also manages to keep the readers guessing regarding the ultimate fate of Douglas and Connie, weaving both remembrance and high jinks through the cities and the art found in them. “Us” is a complicated book, a fitting representation of a longtime relationship in this era where real relationships—especially romances —really are very complicated. If you loved “One Day,” this book will stun you, right at the very end. If you didn’t, then this won’t help at all. A moving and satisfying if somewhat surprising exploration of a contemporary couple’s romance, “Us” is also a strikingly honest portrait of the different stages as well as the different kinds of love.

Available in hardcover from National Book Store.