Gormley stirs up another appetizing concoction: two space aliens, a pair of engaging humans, a serious theme for weight and a dash of satire for spice. Since fifth-grader Elise is forever saving distant planets in her daydreams, she's put out when a UFO lands in stolid, sensible classmate Nick's swimming pool and the antennaed aliens he dubs ""Ace"" and ""Hacker"" take up residence at his house, eating his toothpaste with evident relish. Her annoyance becomes horror when Nick announces that he's leaving with them and that, in fact, he is one of them, stranded as a baby and adopted by humans. He may be right about his origins (he does have odd bumps on his head where antennae would be); but Elise realizes how lonely Nick is likely to be in a culture so foreign. In an engrossing climax, she confronts Ace and Hacker, proving them too alien to understand even the most simple-minded jokes; when Nick wavers, they try (unsuccessfully) to kidnap him. As usual, Gormley surrounds all this with a rather flaky crust of quotidian school and family incident that intensifies the suspense. Illustrations not seen.