An unimaginatively farfetched adventure featuring eleven-year-old ""river rat"" Jelly (short for Jill-Ann), who's happy at home on her Sacramento River delta but annoyed with newcomer Rich, 14, and his constant praise of San Francisco. It's Jelly who dares Rich to board a rickety-looking houseboat that turns up nearby, but he who wants to stay on board when its four occupants turn out to be Alpha Centaurians, disguised as hippies and en route to San Francisco aboard their dual-purpose space-and-river-ship. Jelly finds she has no choice about going along: because they need her as a guide for the 60-mile trip downriver, she'll be held on board until they arrive--and longer, if they don't get there on time. The strangers say they're students, assigned to study the primitive civilization of earth; she thinks they might be delivering bombs, and she makes several attempts to escape so she won't be an unwitting ""junior Benedict Arnold."" But on Rich's urging Jelly decides to ""trust people,"" and once safely at Sausalito (where her brother happens to be moored) she parts from the aliens amicably, even fondly. Readers, however, don't get to know anyone well enough to care, and Parenteau never generates enough suspense to keep them in tow for the trip's sake.