A woman is murdered during the San Francisco marathon and Monk suspects her married lover, Trevor McDowell, even though he was running in the race. Although McDowell disappears from the video tape of the race less than halfway through and reappears only at the end, the data from a computer chip indicates that he was present at all checkpoints during the race. After interrogating the murdered womans ex-husband as a possible suspect only to find that his alibi is probably credible, a frustrated Stottlemeyer provides Monk with a brilliant suggestion--that the computer chip was passed off to someone else during the race. Monk meanwhile has the chance to visit his hero, an aging runner from Nigeria whom Sharona briefly suspects may be the murderers accomplice--a theory Monk refuses even to consider. When Monk figures out what really happened, he must catch the murderer himself to prove his theory.