Actress Sally Larkin is at a small jewelry store trying to sell an expensive piece, but they tell her it's too valuable for them to handle. As she prepares to leave, she says to the owners that she thought she's seen her husband outside. As she goes, they recognize who she is and they go over an article saying she's getting a messy divorce. They hear a scream and run outside to find the car open and Sally gone.
The next day, Stottlemeyer brings a team to investigate the disappearance but they're struck with awe over a double-rainbow. Monk arrives but is the only one uninterested in the rainbow: they're not even. Natalie is sorry at what he's missing. Monk finally gets to the crime scene and goes over Sally's car. Monk is interrupted when his nemesis Harold Krenshaw comes by. Harold is strangely upbeat and admits that he doesn't play competitive game with Monk anymore: he has a new therapist, Dr. Climan. Harold gives Monk the doctor's number and takes off.
Later, Monk and the others go to Aaron Larkin's home. As they wait for the businessman, Disher offers his self-developed gum around. Everyone spit it out as Aaron arrives. Monk is more concerned about Harold's newfound happiness and wonders if hypnosis might work for him. He barely notices that Aaron is lying about not having left his house. Natalie gets him to promise not to see Climan until he talks to Dr. Bell.
That night, Monk is sweeping. A lot. Remembering what everyone said, he makes an appointment with Climan. The next day he meets with the hypnotherapist, who gets Monk to eventually relax. Climan then regresses Monk to before he felt fear. Monk finally goes back as far as it takes.
Stottlemeyer has his men search Larkin's estate while a new relaxed Monk arrives on the scene. Natalie is astonished to see him walk in water puddles and tickle her chin with a daffodil. Monk finally finds something important: a frog. He names it Hoppy, much to Stottlemeyer's astonishment. They go to see Aaron at his office and Stottlemeyer informs him that Susan was selling jewelry and she claimed to be afraid for her life. Aaron has his secretary Emily brings in a list of his properties for the police. However, Monk notices that Aaron was having an affair with his secretary. He was with Emily the night of Susan's disappearance, giving him an alibi. Natalie finally gets Monk to admit he visited Climan, and the detective refuses to say how old he believes he is now.
That night on the road, a panicked Susan runs onto the road in front of a car and calls for the police.
Dr. Neven visits Monk as he eats pancakes and admires his new pet, Hoppy. Neven knows about Climan's reputation and warns that there's nothing he can do for Monk, who is now emotionally six years old. He explains that Monk will eventually snap out of it on his own but there's nothing they can do until then. Disher gets the call about Susan's reappearance. Neven suggests they bring Monk (and Hoppy) along and stick to their normal routine so he'll come out of it sooner.
Aaron is dead at his hunting cabin. Susan is saying that he captured her and chained her up. She finally managed to break free and was forced to beat Aaron to death to make her escape. Monk explains that Aaron abducted his wife and chained her up, but his pants fell down and he killed himself out of embarrassment. Stottlemeyer and the others are a little skeptical. At the police station, they talk to Susan who reiterates her story. Monk is busy flicking pencils but accuses Susan of lying, saying she would have died after three days with no TV. When he takes a piece of gum off of Susan's foot and sticks it in his mouth, Stottlemeyer orders him out despite Monk's claim he can prove she's lying. Monk goes into a temper tantrum and says they'll never see him again: he's going to France. Natalie runs off after him but he gets away with Hoppy.
Monk wanders the streets of San Francisco. He comes across Harold, who has similarly regressed and is arrested for streaking. Monk goes to a playground and fights with the children over the swing set. A mother finally tells him to leave and he notices himself in a mirror plaque. Realizing the woman is right, Monk decides to take Hoppy home.
Meanwhile, Stottlemeyer and Disher drop Susan off at home and assure her there'll be an informal hearing but she's in clear. Once they're gone, she smiles and pours herself a drink… and notices Monk outside with Hoppy.
Driving away, Stottlemeyer notices the lid to Hoppy's box and realizes Monk is nearby.
Monk releases Hoppy at the pond where he found him earlier. Susan confronts him but he boasts that he knew what she did and how she did it. Maturing even as he speaks, Monk explains that she signed the pre-nuptial and regretted it. She set up the disappearance and then hid in the cabin for three days and starved herself for three days. Then she broke into the house, put the rug from the cabin on the floor, and clubbed Aaron to death atop of it. She then rolled him up in the rug, dragged it back to the cabin, and set it up to make it look like he died there. Monk notes she made one mistake: the gum on her shoe was from the gum that Stottlemeyer spit out. She stepped on it when she killed Aaron. He shows her the gum, which is Disher's unique brand. She notes he's been chewing it for over five hours and Monk throws it to the ground. Susan dives after it and Monk struggles with her. Disher and Stottlemeyer arrive and arrest her, and realize that Monk is back to normal.
Later, Monk is back to his usual depressing self and Natalie notes a nest outside with three baby birds. She goes to get a camera at the wonder of life, and Monk tries looking at them. He doesn't get it.