After the events of Super Mario World 2: Yoshi's Island. The couple to whom the stork gave Baby Mario and Baby Luigi allege that the brothers aren't their's. It goes on searching for real parents. They are found by the Yoshis to which Luigi gets kidnapped by Kamek so the Yoshis go to rescue him with baby Mario on their backs.
This game plays very similarly to the other Yoshi Island games with the player playing as Yoshi with access to moves like the flutter-jump and egg throwing. If you get hit, Baby Mario falls off and you have to catch him in his bubble before you run out of stars.
Completing a level is now made easier since it now saves your completeion progress of a level. You don't have to get all the Red Coins, flowers and keep 30 stars in a single run anymore.
The new mechanics in this game are the giant eggs and the transformations. The giant eggs are obtained from eating giant Shy Guys. The metal Variety is obtained through swallowing the Metal Shy Guy. Giant Eggs are giant wrecking balls, destroying everything in its path. Metal eggs obey the laws of gravity and roll along the stage's surface including underwater. While transformations are nothing new for a Yoshi game, there are several new transformations and each are controlled via the 3DS gyroscope.
The music variety is very basic, with all the music in almost all the levels revolving around variations of the same melody. There is much use of kazoos for some reason, as heard in many of the tracks. The game's music was composed by Masayoshi Ishi.
However, this hasn't really stifled sales too much, landing at the No. 10 spot on UK individual SKU Charts of its launch week. In Japan, the game sold 58285 units in its first week, but this is 1/5 of its predecessor's first week sales. In the US, the game sold less than 160 thousand in its launch month.