About the Impel Down Arc
The Impel Down Arc is the 21st story arc of the manga and anime One Piece, and the third story arc of the Summit War Saga.
Luffy puts finding his crew on hold in favor of rescuing his brother Ace from the underwater prison Impel Down. With Boa Hancock's help, he manages to infiltrate the facility. However, fulfilling his objective and escaping the hellish prison might be easier said than done.
Garp visits Ace at Impel Down and expresses his disappointment that both Luffy and Ace turned to piracy when he wished for them to be Marines. Ace counters it would not have mattered anyway since they have the blood of world-class criminals in them. He goes on further explaining that he took his mother's name "Portgas" out of love for her and does not care in the slightest for his father, claiming Whitebeard to be his only true father.
Meanwhile, Vice Admiral Momonga's ship reaches one of the gates, spotting a pirate ship as they near it. Momonga leaves it alone, trying to keep to his schedule of reaching Impel Down. The pirate ship turns out to be Buggy the Clown's as his crew tries to reach Impel Down to save their captain, who has been captured. They are stopped as they get dangerously close to the Calm Belt. The crew argues with Alvida on whether to continue their mission or not, though she reminds them of the impossible nature of it all and seems more interested in taking Buggy's ship for herself. Upon realizing she is right, the crew wishes Buggy good luck and sails on.