In The Immortal's Secret, art merges with magic to explore the seductive allure of eternal life. Dr. Benjamin Mann, a curator at the Louvre who is recovering from cancer, is thrust into an unexpected adventure when his eccentric grandfather, Josiah Mann, vanishes.

Desperate for clues, Benjamin turns to Josiah's treasured possession: a portrait called The Immortal, painted by the Renaissance master Andrea Sartore. In a violent rage, Benjamin rips The Immortal from her frame and finds a golden book-a treasure Josiah has been searching for since his youth.

The discovery leads Benjamin on a journey from Paris to Venice ​during the bubonic plague to the Sinai Desert, where he encounters a host of mysterious characters who hold clues about what happened to Josiah. As Benjamin delves deeper into Josiah's history, the identity of The Immortal, and the secret embedded in her golden book, his journey becomes a crucible, forcing him to confront his life decisions, vulnerabilities, and regrets.