The story starts out with a normal picture of two girls in Hartford shopping for the clothes they'll need for a summer at the beach where they've gone for years. And then April Merriman finds herself caught in a situation she resents; she must accept an invitation from her aunt -- a young aunt- to stay with her in a cabin on a lake in the Maine woods. It is a homesick April who arrives a day before she is expected; but the very chance thrusts her precipitately into friendship with the Olivers, Sally and Kent, whose life hasn't been drawn on easy lines. April does some growing up that summer, with a few adventures thrown in, and promise not only of romance for Aunt Ellen -- but a healthy boy-girl relationship with Kent for April, make a predictable but entertaining tale. The author, however, cannot seem to resist bringing in all sorts of facts about nature and the region in rather stilted conversation. April seems unduly young for her thirteen years at the start of the story; and perhaps unduly mature at the close. And the ""mystery"" of the title is one that involves some moral principles rather than hidden treasure. With these reservations, one can still say it is a good story.