It's late, and Monk finds himself at a seedy, dimly-lit bar in the worst part of town. He's there because a mysterious man has offered him five hundred dollars just to meet with him. Monk sits across from the man in a shadowy booth, barely able to see the man's face. The man introduces himself as Derek Bronson. Monk knows the name. Derek Bronson is a millionaire who has been presumed dead since disappearing seven years ago. Bronson tells Monk his story. He got lost at sea in a ballooning accident, but after surviving the accident, he contracted a horrible disease and decided never to return to civilization. Bronson leans forward into the light, revealing a disfigured and bandaged face. Bronson has leprosy.
Monk runs from the scene. He wants no part of this nightmarish client, despite Bronson's generous offer of $25,000 to take his case. But Natalie, conscious of the money and sympathetic to Bronson's plight, convinces Monk to keep an open mind. They visit a doctor, an expert on leprosy named Aaron Polanski, who reassures Monk that there is minimal risk of contracting the disease. Monk agrees to talk to Bronson for a second time.
Monk goes back to the shadowy bar, where Bronson explains his situation. In a week, Bronson will be declared legally dead, and he fears that lawyers will go through his papers and find love letters that were written to him by a former mistress. He wants Monk to break into his home and steal those letters before his wife, Mandy, sees them. He wants Monk to do it because he knows Monk is an honorable man who won't double-cross him. Monk agrees to take the job.
Monk and Natalie follow Bronson's instructions and break into his large mansion, where they find the letters exactly where Bronson said they would be. Everything is going according to plan until Mandy Bronson appears with a gun. When Monk and Natalie explain why they are there, Mandy demands that they take her to see her long-lost husband.
Monk and Natalie watch from a distance as Derek and Mandy Bronson reunite in a secluded parking garage. Mandy begs Derek to come home, but he refuses. Due to a drug allergy, Derek's condition can't be helped, and he is too ashamed to be seen in public looking the way he does. During their meeting, Mandy reveals that once Derek is declared legally dead, Derek's nephews would inherit his estate, and Mandy would be forced to move from their home. This is not what Derek wants, so he asks Monk to attend an upcoming probate hearing and testify that Derek is still alive. Monk agrees.
Then, something strange happens. The night before the big probate hearing, at the Bronson mansion, an unknown man is playing the piano for Mandy Bronson. Mandy walks up to the piano player, and calmly shoots him dead.
The next day, Monk testifies at the probate hearing that Derek Bronson is still alive. The judge agrees, and rules that Mandy Bronson will continue to control Derek Bronson's estate. Meanwhile, Capt. Stottlemeyer and Lt. Disher are investigating the case of a missing piano player. Interestingly, they find a makeup kit and a book on skin diseases in the dead piano player's apartment. They also find that the victim had recently played a private party for Mandy Bronson. Then, later that night, while Monk is babysitting for Julie, Monk makes a realization. When Derek Bronson told Monk how to break into his mansion, Bronson knew exactly how to disarm the mansion's security system, despite the fact that the alarm system was clearly installed after Bronson supposedly disappeared. Monk has been duped! As Monk is making this realization, Natalie bursts in. She's just returned from a date with Dr. Polanski, who just revealed that he too is a leper. The doctor is completely cured, but after kissing him, Natalie still feels the need to wash her mouth out with antiseptic. Suddenly, Monk and Natalie have a lot in common.
Monk and Natalie return to the Bronson mansion, where Monk explains what he thinks is going on. Derek Bronson is not a leper. He was killed seven years ago by Mandy, who has been living off his money ever since. With Derek about to be declared legally dead, Mandy stood to lose control of the estate, so she came up with a plan. She arranged for somebody who resembled Derek to "suddenly reappear." The fake Derek disguised himself as a leper and contacted Monk, who, with his fear of germs, would be the perfect patsy: an honest man who wouldn't get too good a look at the imposter. With Monk's testimony, Mandy could make the case that Derek is still alive, and thereby maintain her control over the estate.
As Monk and Natalie hop over the wall onto the Bronson estate, they hear a hissing sound. They sneak around back to find Mandy Bronson inflating a hot air balloon. Mandy leaves the balloon for a moment to take a phone call. Monk and Natalie run to the balloon, where they find the dead piano player, who Monk identifies as the Derek Bronson imposter. Mandy was about to take off and dispose of the body! But there's trouble. Mandy has returned, and she has a gun. With nowhere to run, Monk and Natalie take off in the balloon. Mandy shoots and hits the balloon, which starts to fall from the sky. The balloon hits the ground, throwing Natalie from the basket. But Monk remains inside as the wind blows the balloon over a cliff. Monk manages to grab onto a branch, holding on for dear life to the side of the cliff. A hand reaches over. Monk is about to take hold, when he realizes the hand belongs to Dr. Polanski – the real leper! Polanski and Lt. Disher had become suspicious of Mandy Bronson, and they arrived together at the estate just in time. But in order for Monk to be saved, he must take Polanski's hand. After some thinking, Monk reluctantly decides that touching a leper is preferable to falling from a cliff to certain death.
In the end, Mandy Bronson is brought to justice. Monk comes away from the case still holding onto his fear of lepers, despite owing his life to one. Natalie, on the other hand, decides to take a step forward and overcome her fears by going on another date with Dr. Polanski.