In July 1794, he visited Jules and Jean to inform them that a stranger, Ian, was looking for Jules at the sign of Le Chien Gris, an inn often visited by Jules and Jean. He offered to get a physician for Susan Foreman, but it was a trap, and the physician turned Susan and Barbara Wright back in to the jailer of the Conciergerie.
Jules arranged a meeting between Léon and Ian, not knowing it was a trap for Ian. Léon captured Ian and questioned him, but didn't believe that Ian came to France in a small box from 1963. Jules came to rescue Ian, and killed Léon and his soldiers. (TV: The Reign of Terror)