Sherlock Bones and Dr. Catson are entangled in an arithmetic adventure. They must follow a teasing trail of addition and subtraction-themed maths puzzles, and it's up to the reader to solve each sum, all in the hope of foiling Professor Moriratty's evil plan. This charming maths activity book features 'Sherlock's Secret Solver' magnifying glass which reveals the answer to each puzzle when placed over a moiré area, allowing children to test their numeracy skills again and again.