Monk knows arrogant local furniture magnate Trevor McDowell threw his young mistress, Gwen Zaleski, off her 21st-story balcony. Trouble is, McDowell was across town running in a citywide marathon at the time of the murder. His alibi is perfect: the runners were tracked by microchips secured to their shoelaces during the race, and the computer shows McDowell ran the full 26 miles.