Paris, 1777. The philosopher Jean-Jacques Rousseau receives a mysterious letter from a foreign visitor, Gavin Mulvany, asking whether the great man will take walks with him. Against his better judgement, Rousseau agrees. Might this strange man, who claims to be from the 21st century, be the true friend that Rousseau has been searching for his whole life? Paris, 2022. Gavin, a middle-aged academic, leaves his husband behind in Ireland to finish a long-delayed biography of Rousseau. While in Paris, he avoids work on his book by instead taking walks with Rousseau himself. As they wander the crowded streets of the eighteenth-century city, Gavin and Rousseau open up about certain past actions that have come to define them. Was Rousseau justified in abandoning his children? Should Gavin be forgiven for the terrible crime he committed to protect a man he once loved?