In their further efforts to recover the historical artifacts lost when they set fire to Merlin's office, Ash and Grant go back to the early 1930s, where they must infiltrate Alcatraz Prison and secure a locket belonging to Al Capone. They find themselves at odds when Ash plays the role of a prisoner, while Grant is a guard. Capone takes a liking to Ash, whom he sees as a younger version of himself, and places Ash under his protection.

Before they can return to their own time, the boys must help foil an escape from the Rock, expose a corrupt and dangerous guard, and secure the pendant.