Three homeless men are singing Christmas carols in an alleyway and wondering where their friend Willie is. They hear Willie as he runs down the street screaming in terror. He says some guy is chasing them but there's no one behind him. A car pulls across the other end of the alleyway and Willie goes running away.
The next morning, they go looking for him and find Willie's cart… and Willie, dead in a refrigerator.
Monk is decorating his apartment with a cardboard Christmas tree and unhappy about the holidays as Natalie prepares for the holiday with her daughter. Natalie is preparing soup for Captain Stottlemeyer, who is having back problems. The three homeless men arrive and Monk goes into a panic, but they finally explain that they want to hire him to investigate Willie's death. The police say it was an accident but the men insist he was murdered. Natalie invites them to stay for a meal and Monk to take the murder. He balks until they start looking around and he takes them to the crime scene just to get them out of his apartment.
Monk examines the crime scene from his car and notices the lack of handprints, indicating that Willie was dead before he was put in the refrigerator. They go to talk to Stottlemeyer only to find Disher in charge of the station. He quickly agrees to reopen the case and explains that Stottlemeyer is in his office but is refusing to go home. Stottlemeyer is talking to a couple who are making a report that God painted a drawing of a fountain and the word "drink" on their door. They believe it was a sign from God, and the wife was ill until she went to the fountain at a monastery. She was cured of her disease. Stottlemeyer then goes to his office where Monk explains that the doctors haven't been able to do anything for him. He wants to go over the open cases but Disher shows him all the completed case files. Stottlemeyer, depressed over his son not having called and the lack of a date in two years, prepares to go home after drinking a bottle of an herbal chalk extract, and talks about how bad is life is. As he arrives home, he finds a drawing of the fountain on his door.
At the monastery, Stottlemeyer sees other ailing people. He talks to the first woman who was cured, Kate Doyle, who says she saw the sign and spotted the fountain during a tour group. Stottlemeyer approaches the fountain and Brother Andrew notes that they're losing money: they were planning to build a schoolroom where the fountain is. He suggests Stottlemeyer try a leap of faith and the captain takes a drink.
Monk is trying to cash in the bottles that the homeless men paid him with and discovers that three of the cans are from Canada. He notices Natalie has bought items for a meal, and realizes that she's invited the three men to dinner. Meanwhile, Stottlemeyer stops by his pharmacist, Owen McCloskey, who has heard of the fountain, and notes that he isn't feeling better yet. Stottlemeyer notices a crucifix on the wall, and the pharmacist explains that his partner hung it up and then disappeared after embezzling money from the pharmacy.
As Natalie prepares to have company at Monk's apartment, Stottlemeyer wakes up and discovers that he's able to walk unassisted.
The three homeless men begin the dinner with grace while Monk fans an air freshener around the room and tapes the doorways closed. They discuss the case and Monk explains that Willie was murdered and the case has been reopened. Monk tries to get them to pay the extra 0.15 from the bottles and realizes they're Stottlemeyer's herbal back pain: Willie got them from the captain's dumpster. They go to tell Disher and realize that he's acquired a mustache to go with his job. Disher explains that Stottlemeyer has left and didn't say how long he'd be gone.
Stottlemeyer goes back to the monastery and leaves his cane and his prescriptions in a pile where the others who have been cured have left their crutches and medicine.
Later, Monk and Natalie visit the monastery, tracking Stottlemeyer there after he left his car outside. Natalie suggests that Monk drink from the fountain. Monk doesn't believe it and talks to Kate, who recognizes their photo of Stottlemeyer. She directs them inside where they run into Brother Andrew. He refers to Stottlemeyer as "Brother Leland" and says he's taken a vow of silence. Only Monk is allowed past the foyer and Monk goes up to the library and finds a clean-shaven Stottlemeyer. Monk uses Charades to try to explain about the murdered homeless man. Stottlemeyer doesn't know anything about the murdered man and tries to gesture that he's found God. He gives Monk a package with his badge and a note explaining that he is leaving on Christmas Day on a two-year overseas mission. Monk is shocked, unsure what he'll do without his friend. Natalie again suggests that Monk drink from the fountain but he refuses and she wonders what he really believes in. She takes him to the monastery to get a drink.
At the pharmacy, McCloskey confronts Kate, who is taking pills. He says they can't be seen together and she says she can't do it anymore because some of them are really sick. He slaps her, saying they both know what's under the fountain. She reluctantly agrees and he kisses her, saying it's almost over.
Monk and Natalie arrive at the fountain and Natalie offers him a drink. Monk refuses and notices the pile of crutches and canes. He also notices all the medicine bottles… and the fact that more than half of them are from the same pharmacy. He takes several of them and Kate arrives, and Monk explains it's a crime scene. She confesses, saying her fiancé McCloskey caught his business partner stealing money and killed him.
Monk and Natalie sneak into the monastery disguised as monks and manage to get to the chapel. They find Stottlemeyer and Monk chants out the explanation: McCloskey killed his partner nine years ago and buried him on the monastery grounds. Then the monks built a fountain on the site and nine years later, read that the monks were building a schoolroom on the site. McCloskey used the prescriptions to make his patients sicker, and then painted the fountain on the doors of his patients. When Willie saw him painting the sign on Stottlemeyer's door, McCloskey had to kill him.
Outside, Stottlemeyer realizes that the entire thing was a hoax and the monks wouldn't dig up the fountain once it was considered a holy place. McCloskey is arrested and the homeless men thank Monk and give him a jar of homemade gravy. They pat him on the back… and Monk leaves them his coat. Disher talks to Stottlemeyer, who says he's not sorry for what he went through. He felt that he could start over and he resolves to call his son Jared… and gives Disher a razor.
Later, Monk goes to the monastery and looks at the fountain. Crossing the police lines, he takes a glass and fills it with water. He considers the glass and considers the fountain…