Why did Papa offer me to the Lord to become a preacher? Samuel Grant wonders. "My brother Toby would be much better than me. Things come easier for him than they do me."His sister Hallie's confession to taking the red string off the wrist of the older brother when the twins were toddlers adds more confusion to Samuel's dilemma. Hallie doesn't know if she put the string back on the right brother's arm. Now what will happen?"Maybe I'm not Samuel. Maybe I can live a normal life and get married to Katrina."
The brothers live in a state of uncertainty while waiting for their true identity to be uncovered. On the way to finding out which twin is the older one, the Grant siblings endure hair-raising experiences, such a helping birth a baby, concealing the mother's whereabouts, caring for the newborn until the mother decides what she'll do with the infant, and an impromptu wedding. These and more challenges strengthen the character of the young man who was actually offered to the Lord in Papa's Vow.