It is, it is! Down at the dock he was in a disguise! We've got a mystery,"" whoops Dash's younger brother JC when the two children notice that Dan Alexander, the blond surfer in the house next door, is the same man as Jason Herrick, the dark, bearded ""lawyer"" they'd met earlier on the dock. It's a matter of posture and elbows, which only JC, who mimics people, and talented cartoonist Dash are likely to pick up on. That settled, they do more snooping and sketching. The surfer sneaks in and out at night with a fake scuba tank; his Herrick disguise turns up in flitty Mrs. Guizot's spare closet', and the children are investigating his house when he catches them, ties them up. . . and is stopped just short of doing worse by their teenage sister bursting in, feet and fists flying, and knocking him out. It's a tame but likable adventure, with the suspicious behavior exposed at last as a smuggling operation, involving the theft of Mrs. Guizot's artist-father's paintings from a French museum whose possession of them she resents. Campbell makes good use of the Fire Island setting and communicates something of JC's delight in a mystery, so that addicts attracted by the title won't be disappointed.