"Once Upon a Prince" follows Nathaniel, the crown prince of the fictitious European country of Cambria, as he spends his last 'real holiday' in Georgia incognito at June Waller's island estate.
During his stay, Nathaniel, known as 'Nate', helps out local gardener Susanna after she gets a flat tire. He offers to assist at her family nursery while her father is in the hospital recovering from a fall.
When the press discovers Nate's true identity, he returns home but arranges for Susanna to be hired by Waller. He also invites her to attend his coronation, as he has developed a crush on her.
However, the queen still hopes to match Nathaniel with a suitable aristocrat, particularly Lady Ginny. She tries to break up Nate and Susanna by hiring June to assist famous Sir Edward Tilbury in redesigning the palace's private Florentine gardens.
Nathaniel is torn between his duty to his nation and his feelings for Susanna. He shows determination to fulfill his responsibilities but refuses to blindly obey traditions like marrying a fellow aristocrat.