As anyone can guess from the title, this tells how the girls get themselves admitted to Marty McGee's space lab. The lab is Marty's room and the girls his younger sisters--Rachel, the middle one, and Jenny, still in a playpen. At first Rachel is refused entrance, but diapered Jenny sneaks in later, dons Marty's helmet, and then, to Rachel's and Marty's astonishment, flies out the window, does a few loops around the house and the room, and lands, lightly, back in her playpen. Inevitably, Rachel strikes the bargain: she'll find out how Jenny did it if Marty will remove the ""no girls"" sign on his door. The ending is a real coup, for the girls and for Alexander: it's Jenny's baby rattle that catalyzed her flight; but when Marry tries the trick he finds that he's just too heavy to get more than a few inches off the floor, even With the rattle. From the beginning, the expressions of all three children--especially Jenny--give the transaction the amusing, appealing, recognizable quality we count on from Alexander.