When Claire and Ethan made a pinky promise at seventeen to marry each other if they were still alone at thirty — they never thought they'd actually have to go through with it.
But thirty arrives, and so does the wedding. What starts as a joke between best friends slowly becomes something neither of them can explain away. As Claire's career spirals into the orbit of Ethan's calculating mother, and a charming ex threatens to rewrite the past, they're forced to ask whether a promise made over a diner milkshake was ever really just a joke.
