Partly because her widowed mother believes that she should get out and do things instead of living in a ""dream world,"" Lark is sent off on a five-day sailing trip with her brother Joe and Gram's Northwest Coast neighbor, silver-haired Skipper Perry. There is some rough sea and Lark is inconveniently clumsy on board, but the real excitement begins on Lucy Island--where birds swoop dangerously at night, where Indian spirit tales seem compellingly real, and where a treasure trunk from an old wreck might possibly turn up. Once on the island Lark meets lighthouse keeper's son Andy, a boy her age who shares her sense of mystery. Andy suspects a fat stranger of something fishy, and when the two children learn of a nearby theft of valuable Indian carvings they decide to snoop him out. They're right about the man, and as the story progresses the spirit-world atmospherics are dropped in favor of some down-to-earth crook-catching--with a heavy overload of developmental gains at the end: in the last two pages Lark concludes that it's what you do about the Other World that counts, that she's not a weirdo (or too tall), that she no longer needs to escape into her imagination as ""reality could be pretty exciting,"" and that she might even warm up to the gym teacher (Mother's boyfriend?) whom she's resented all along. In addition--and it is--Harris appends a chapter telling the pointless and unrelated life story of the Lucy that the island was named for. Clearly, the author knows and loves the territory she describes, but here she uses it only for authentic local color in a fairly ordinary mystery-adventure.