As Laura heads out to summer camp, Hamtaro and Oxnard come across a young girl by the name of Haibi, who's trying to find the ocean. Haibi found a treasure map while searching through the storage burrow and is now willing to find the treasure to help her heat exhausted granny. The Ham-Hams, who are more than willing to help her, pack their things and set out to find the treasure.

Source: Wikipedia

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Prequel: Hamtarou no Otanjoubi: Mama o Tasunete Sanzen Techi Techi
Sequel: Ham-chans to Niji no Kuni no Ouji-sama: Sekai de Ichiban no Takaramono