Aokiji lost his life while fighting his friend Akainu only to woke up in a grave after meeting a strange man by the name of Hagoromo.
Aokiji learns that he transmigrated into the body of a teen by the name of Haku who died protecting his master.
He learns that even as Haku he is able to use Ice power, but it is not only weaker than his former power, but he also lost the logia intangibility, but lucky for him in return he gains some part of Hagoromo chakra and a strange System that gives him a secret inventory and helps him to level up his Ice Release, three Haki and Rokushiki to levels that one has ever reached before.
Haku(Aokiji) knows that he still weak and being alone is going to get him killed, so he decides to return to Kiri to become stronger under the protection of the new Mizukage, when he is strange enough he is going to bring justice to the shinobi world like his did in his previous one.