It seems like just another day in the park for Janet Novak and her playful group of foster children... until her toddler Tommy goes missing. The police rush in to search for the child and soon find the boy safe and sound, but with a little surprise in hand -- he's carrying a human finger! Realizing Tommy is too young to tell them where he found it, Captain Stottlemeyer locks down the park and begins searching for a body. When the police don't find anything, Stottlemeyer calls on Adrian Monk to put the pieces together.
Monk tries to question Tommy in hopes he may provide a clue as to where he found the finger, but the only thing Monk learns is that he's quickly grown fond of the little boy. When a social worker decides that Tommy should be removed from Janet Novak's care, Monk surprisingly volunteers to take temporary custody of him. Unable to develop any leads from his time with Tommy or witth Janet Novak, Monk turns to the severed digit for more clues, and notices that a callous and rosin residue on the finger are sure signs that it belonged to a violin player.
Monk and Natalie begin looking for violinists who have gone missing. After a long search, Monk finally comes upon something strange at the home of Daniel Carlyle and his mother Abigail. Daniel is a professional violinist - but his hands show no signs he's ever played the instrument. It's an apparent dead end at first, but a little snooping reveals that the "Daniel Carlyle" they met was in fact an imposter.
Monk and Natalie trail the Carlyles to find out what they've done with the real Daniel, and they soon learn that the real Daniel has been kidnapped... but not by them. Someone else is holding Daniel hostage and trying to extort money from the family!
The Carlyles have been told by the kidnappers not to go to the police, so Monk agrees to help them, but when he's sent to a rooftop for a money drop with the kidnappers,he accidentally gives the money to the wrong person. The Carlyles have no choice but to call in Captain Stottlemeyer and Lt. Disher to recover the money and join the case.
Meanwhile, Monk is growing more and more attached to Tommy, and even tells Doctor Kroger that he's thinking about adopting him. Then one day, while Monk is reading a story to him, Tommy provides the key clue that cracks this case wide open when he gets into Natalie's purse and takes a lipstick. Eureka! Tommy must have gotten the severed finger from Janet Novak's purse, which means it's Janet Novak who kidnapped Daniel Carlyle! Sure enough, the police arrive at Novak's house and find the captive violinist.
With the case solved, Monk realizes he needs to give custody of Tommy to parents who can better take care of him. That way, he can focus on what he does best: solving crimes.