Many things happen to Mark the day he receives a glove and ball from Uncle Marry. For one thing, the older boys allow him to participate in their game. For another, Mark hits a triple. But the day had its catastrophic aspects as well. A tiff with a jealous teammate leads Mark up a tree in search of his new glove and provokes a heroic retrieve by the Fire Department. Another hit finds its mark among the flower pots of a neighbor and an ignored pet makes his own brand of mischief. Nevertheless nothing can detract from the importance of the major occasion as another boy joins the ranks of the baseball world in reality and in his new-found fantasies. The boyish quality and enthusiasm for the sport come through enhanced admirably by Paul Galdone's wholesome pictures in full color, but the story line is only adequate, not inspired.