Mario is not around when the new threat arrive. Both brothers are busy living their life back in Brooklyn New York. Princess Toadstool would have called for them, except that there are so many enemies around. He appointed Yoshi to send the message to them and bring them back to help the Kingdom. The princess know that he can defend on Yoshi since the latter has worked with Mario before to defeat the threat King Kuppa. Now, that a new threat is here, Mario is needed once more, but before that, Yoshi need to pass the threat and make contact with Mario.
Sure, this would be easier for Mario, but since Mario is not around, Yoshi need to defend on himself. His mission is important and it would really be a big help if he is able to bring Mario back with him. Yoshi knows the land since he was born in this place and he is the best candidate to do the job since aside the brothers, he is the only one that has done battle with King Kuppa before. Yoshi is also agile, aside from the fact that he is capable of using some of the great powers that can be acquired in all the land.
In this game, the Mario brothers are not around and you have no choice, but to play as Yoshi. The game is played like the old Mario game, except to the fact that you would be Yoshi in this game. The enemies are not the same as the new villain in this game brings new adventure for you. You have three life in this game and each time that you are touched by an enemy, you lost one of them. You would lost once all of your lives are gone, but you don't need to worry if you have life left since there are some life bonus that you can grab.