S10E353: A Man's Promise Never Dies!! To the Friend Waiting Under the Distant Sky

Usopp, Chopper, Robin and Franky return to the Sunny, discovering their ship had been ransacked before waking up Luffy, Zoro and Sanji. After informing the three about their current status and Nami's abduction, Franky reveals that he asked Brook before they parted way of is what driving him as he could not really return to society due to his appearance. Brook reveals that he and his crew made a promise to return to Reverse Mountain where they left a valuable friend fifty years ago. Luffy, Zoro, Usopp and Sanji are astonished to find out that friend Brook referred to is the whale Laboon.