An angry Willie Nelson accuses his road manager, Sonny Cross, of embezzlement just hours before making a San Francisco radio appearance. Sonny later arrives at the radio station to find a note summoning him to a side entrance. As Sonny disappears down an alley, two shots ring out, and an engineer throws open the side door to find a blind woman, Mrs. Mass, screaming hysterically - and Willie Nelson hovering over Sonny's dead body.
An injured Captain Stottlemeyer decides to put Lt. Disher in charge of the investigation, and Disher loses no time in calling in Adrian Monk. Monk learns that only Sonny, Mrs. Mass, and Willie Nelson were in the alley, and even though he seems to be the most likely suspect. Monk can't picture Willie - a favorite singer of his late wife Trudy - as a killer. Stottlemeyer arrives with his right arm in a sling. Mrs. Mass gently shakes his left hand, then identifies Willie's voice as the one that threatened to kill her if she spoke to the police. Orphaned at 16 in a car accident that also robbed her of her sight, Mrs. Mass is a persuasive witness. Things look grim for the Red-Headed Stranger.
With Sharona gushing about her great new boyfriend Justin, and the SFPD occupied with trying to track down a persistent and elusive streaker, Monk starts to investigate. He begins by learning more about Sonny Cross. A reckless womanizer and boozehound, Cross had previously served two years in prison for vehicular manslaughter. Willie had been close to firing Cross on many occasions, but never had the heart.
For now, Willie Nelson is still the prime suspect, and the police arrest him and formally press charges. Meanwhile, Monk decides to go see Mrs. Mass again. He learns she received a concussion after falling in a wet supermarket aisle the year before, but never sued. As they say goodbye, she offers to shake his hand, and Monk suddenly remembers how she'd previously shaken Stottlemeyer's hand - and offered him her left hand because his right hand was in a sling. How had she known that... unless she had somehow regained her sight!
The SFPD has finally caught the streaker, and Monk bails him out of jail so he can lay a trap for Mrs. Mass. From a place of concealment, Monk and Sharona watch Mrs. Mass turn her head in wonder as the nudist streaks by her - and Sharona recognizes the streaker as her new boyfriend, Justin!
Once in custody, Mrs. Mass finally comes clean: the fall last year in the supermarket somehow reconnected her optic nerve, restoring partial vision in one eye. She'd kept this miracle to herself in order to be above suspicion when she took finally her revenge on Sonny Cross - the drunk driver that took away her family and her eyesight 30 years before.
All charges against Willie Nelson are dropped, and on a crisp autumn sky under a sparkling blue sky, the two new friends play a soulful, moving duet together - Willie on guitar, Monk on clarinet - at the site of Trudy's grave.