On a gray winter's day on Sparrow Island, Abby's sister Mary is awakened from an afternoon nap by a ringing telephone that she doesn't reach in time. Minutes later someone breaks into her home! Upon seeing Mary and her dog Finnegan, the intruder flees, leaving Mary with only a glimpse of him. Abby immediately begins searching for clues about similar break-ins occurring all over the island, and Mary offers her valuable scrimshaw collection as bait in an ingenious trap to catch the thief. In other goings-on, young Bobby McDonald joins a stand-up comedy competition, and the usually staid George Stanton gets caught up in a ruckus over banned doughnuts at the senior center. As Abby is pulled deeper into the mystery, she is reminded that tru treasure lies within.