Other source:

Tienshinhan, Krillin, and Yamcha ask Master Roshi why he is not teaching them anymore and he explains to them that he has nothing more to teach and that they should seek training on their own. Mr. Popo sends Goku to a forest in search of someone who can train his mind. Goku meets a large family that lives in a cabin and the grandfather of the family, a simple fisherman, turns out to be able to teach Goku more than he expected.