Saint Denis is a suburb (banlieu) just north of Paris. The martyred saint was said to have been beheaded in Paris, yet miraculously picked up his own head, washed it and walked nearly ten kilometers north to his future namesake village. In Paris, you can see his headless statue by the main door of Notre Dame and his headless portrait in the mosiac of Sacre Coeur.
The city grew around the Basilica of Saint Denis, where his relics are currently stored.
Line 13 of the Parisian métro connects Saint-Denis to Paris, at the Basilique de Saint-Denis stop. No additional fare is required.