Hidden under the cloak of the Amazon Queen, Luffy heads to Impel Down, the great underwater prison, to free his brother Ace! But on the way, he runs into an old acquaintance: Buggy! The latter resolves to team up with him to increase his chances of escape, but each level of the prison is guarded by more monstrous guards than the last.
Meanwhile, an unprecedented mobilization of Marine forces is taking place in preparation for a titanic confrontation with 5 pirate captains, gathered for the first time in Mariejois!