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.
Stottlemeyer's theory: McDowell used an accomplice, getting another runner to carry both his own and McDowell's microchip. Monk visits the marathon office looking for a runner whose running time sheet perfectly matches McDowell's, but there isn't one. However there is one runner whose times nearly match McDowell's: the great Tonday Mawwaka of Nigeria, a legendary marathoner and Monk's boyhood hero.
Sharona laughs when Monk tells her used to be a track and field star in high school - until his burgeoning O.C.D. symptoms cost his team a championship (Monk was obsessed with fixing his uneven shoelaces at the beginning of a race). Monk then visits Tonday. They bond instantly, but Monk can't dismiss him as a suspect. They review videotape documenting yesterday's race (which Tonday says was his last), during which a news crew recorded every inch of Tonday's valedictory run. In the video, Monk sees McDowell keeping pace with Tonday for several miles, then dropping off-camera for the bulk of the race, then suddenly reappearing a few feet ahead of Tonday for the finish.
Although Gwen's shifty ex-husband has already been arrested, Monk is more convinced than ever that it was McDowell who killed her. Retracing the marathon route, Monk finds the exact place where McDowell disappeared from the video. He speculates that McDowell left the race, changed clothes, paid his deadly visit to Gwen, than rejoined the marathoners at the finish line.
Monk confronts McDowell with his theory that McDowell murdered Gwen after she threatened to tell his wife about their affair. He shows McDowell "before and after" video stills of McDowell at mile six of the marathon, sweating heavily, and then at the finish line, hardly perspiring. McDowell laughs, knowing the computer readout proves that he never left the race and that the computer trumps everything. Frustrated, Monk goes home and watches, again and again, the video of Tonday's run... and finally figures out what happened.
McDowell's chip never left the race, but it wasn't with another runner - it was with the video crew documenting Tonday's run. McDowell had it hidden on the camera cart! Monk and Sharona race to the news station, arriving seconds behind McDowell, who seizes the hidden microchip and takes off running. Monk's old marathon skills return as he chases after him. With incredible force of will, Monk overtakes McDowell, tackles him, and recovers the incriminating microchip. It was Monk's most important race since high school - and this time he won!