Unceremoniously left by his parents at Arthritis Hall to stay with his Great Aunt Harriet, Duncan expects his summer holidays to be very dull. Instead, he finds himself at odds with a criminal group of octogenarians intent on worldwide thievery. When he meets Ursula, the caretaker’s daughter who knows every hidden route and secret passageway in the building, things get even more dangerous, as the two get dragged into a creepy plot involving goggles, knitting house-breakers and some bizarre inventions. Duncan could never have imagined the summer holidays would turn out like this!