Tottoko Hamtarou OVA 3: Hamuchanzu to Niji no Kuni no Oujisama - Sekai de Ichiban no Takaramono

ハムちゃんずと虹にじの国くにの王子おーじさま 〜せかいでいちばんのたからもの〜

Based on the third Hamtaro game (fourth in Japan), Prince Bo falls from the sky when the rainbow disappears. Now Hamtaro and the Ham-Hams must help him bring back the rainbow.