Colin the Crab, the most skillful builder on the eastern riverbank, never hesitates to help his friends. Now Colin is busy with his own new project-a garden pavilion for his home. But after a week of hard work, the pavilion of his dreams is still unfinished. Even worse, a boisterous fish family has taken over the construction site. The exhausted Colin buries himself under a blanket and refuses to open his curtains. Puzzled, Colin’s friends call an emergency meeting-it’s time for them to take action!

Carwyn y Cranc yw adeiladwr gorau ardal dwyrain yr afon, ac nid yw byth yn oedi i helpu ei ffrindiau. Ar ol wythnos caled o waith, nid yw'r pafiliwn las o'i freuddwydion dal heb cael ei orffen, ac mae yna deulu o bysgod swnllyd wedi cymeryd drosodd ei safle adeiladu.

Wedi ei ymladd, mae Carwyn yn swatio dan ei flanced ac yn gwrthod agor y llenni. Mae ei ffrindiau'i gyd wedi drysu'n lan, a felly mae cyfarfod brys yn cael ei drefnu. Mae'n amser i'r anifeiliaid creu cynllun.