Five college friends have arrived at forty in very different circumstances, but with at least one thing in common: they are among the more privileged in society. Elizabeth and Sara are lawyers, Martha is a doctor, Carmen is a wealthy and well-educated homemaker, and Heather, the most successful, is a famous tech executive—and after more than two decades of friendship, they know one another better than anyone.

Then Heather writes a women's advice book detailing the key life "mistakes" of her four friends—opting out, ramping off, giving half effort, and forgetting your fertility—that becomes wildly popular, and Elizabeth, Sara, Martha, and Carmen all feel the sting of Heather's cruel words. Despite their status, these women face everyday obstacles, including work problems, parenting challenges, secondary infertility, racism, sexism, financial stress, and marital woes—and as they weather their fortieth year, each one can't help but wonder if their life might have been different if they had followed Heather's advice.

But as these friends are continually reminded, life is complex, messy, disappointing, and joyful, often all at once—and no one can plan her way out of that reality. In the end, all five women must embrace the idea that their lives are shaped not just by their choices but also by how they handle the obstacles life inevitably throws at us all.