Calliope Silversmith has always had just two friends, Ginger and Noah, in her small Pennsylvania town, and she's fine with that. She's never wanted anything more than her best friends, her moms, their house in the woods, and their family-run yoga studio—except maybe knowing who her sperm donor is. Her curiosity has been building for years, and she can finally find out this summer when she turns eighteen. But when Max and his family move into the house across the woods from Calliope, she realizes it's nice to get to know someone new, so nice that she decides to break her no-dating rule. The stability of her longtime trio wavers as she and Max start to spend more time together. When Calliope finally finds out who her sperm donor is, she learns a truth more shocking and unfathomable than she could have ever dreamed: Her donor is Max's father. How is this even possible? As she and Max struggle to redefine their friendship, Calliope hopes that they can turn a horrific situation into something positive by recognizing and accepting that family isn't just the one we are born into—it can also be the one we choose to make.