The central conflict of Mercy is presented by the case of a man who kills his terminally ill wife as per her wishes. This is an act born out of the deep love he has for her, coupled with the opening scene of the book in the Prologue, which indicates tension within a romantic relationship. This points to one of the central themes in the book: the nature of romantic love and how it impacts the dynamics and power balances within a relationship. This theme is explored largely through the relationships of three couples in the story: Cam and Allie, Jamie and Maggie, and Cam and Mia.

Cam and Allie are the first couple to be introduced by name in the book, and from the very beginning, it is clear that, besides being very different from each other, the love is not equally matched between them. Allie is conscious of the fact that she only managed to marry the extremely handsome, popular, and well-regarded Cameron MacDonald because she was the only other person of their age to eventually return to Wheelock. Significantly, their returns to town were prompted by vastly different motivations—Allie wanted to come back, whereas Cam was obligated to do so.