Mark and Mary's meet-cute happens at a neighborhood drug store, where she is busy buying a pregnancy test, and he has to make an effort to remind her they met in college. After asking her out, he accompanies her as she takes the test, and upon it showing negative, she agrees to a date. One year later, they have fallen deeply, passionately, head over heels in love, and their whirlwind romance leads to marriage. So when Mary suddenly requests they open their relationship to “ethical non-monogamy”, Mark is taken by surprise, but agrees to try it. And that's where this story really begins.

Over the course of a summer, two teenage girls develop the perfect kindred spirit friendship, with one big problem: one of them is dating the other's ex.

When Elliot, a brash 23-year-old living carefree in New York City, meets the sensible Mia and receives a damning diagnosis all in the same week, his world is turned completely upside down. But as their love blossoms amidst the chaos of his treatment, they discover that Elliot's illness is not the real test of their relationship – it's everything else.