For as long as Halley Steen has known her husband Nathan, he has carried a handful of stones in his pocket. Each day he uses those stones to remind him to follow the Golden Rule, moving a stone from one pocket to the other with each act of kindness. So it's not unusual that Nathan stops to help a stranger on the side of the highway while on his way to his son's football game one Friday evening. But that one act will change all of their lives forever, when a car hydroplanes off the road, killing Nathan instantly.

As Halley and her children Ty and Alice struggle with their grief, Nathan's spiritual legacy lives on. A Facebook page appears, where countless stories about Nathan's selfless acts are shared. But among them is one that stands out, from a woman who says that Nathan saved her life. Neither Halley nor her children have ever heard of Madeline Zuckerman. But soon Halley discovers years of e-mails from this woman to her husband on his computer that refer to "our little girl." How could her husband have kept the secret of this other child for their entire marriage? Why had he lied to her? Was he not the man she thought he was?

Only thirteen-year-old Alice maintains unwavering faith in her father. She knows there's an explanation. When she sets out to find Madeline and learn the truth, she will start to unravel the complex story of The One Good Thing Nathan Steen did that had the greatest impact of all.