The game involves an extensive plot. One day while playing outside, Hamtaro and Boss are suddenly witness to a great rainbow. Off the rainbow slides Prince Bo. The Prince states that he can make rainbows by using his umbrella. He tries to prove it to the disbelieving Boss, but realizes too late that the colors on his umbrella are gone!
And thus, Hamtaro and his Ham-Ham friends must set off on a journey in order to collect things that are the proper colors for Prince Bo to use for his umbrella so he can create a rainbow to return home. Certain minigames are needed to reach the colors, and the player must have certain Ham-Hams in their party in order to play them. For example, Sandy is needed to ride pidgeons, and Bijou is needed to collect falling petals, whilst Penelope is needed for rolling on top of cans.
In addition to the original Ham-Hams, the game contains some additional characters, most of which were never seen in the English anime. These include Prince Bo, who is the prince of Rainbowland (a castle that is made of clouds); Flora, or Nurse-Ham; Tux; and various other characters.