Mr. Monk Gets Hypnotized (2008)

Monk runs into Harold Krenshaw and appears to be better. He tells Monk he went to a hypnotherapist. Monk goes to see the man and hypnotizes Monk. When awaken Monk is acting like a kid because that's when Monk was asked was the happiest time of his life. Now Monk is acting like a kid while investigating the disappearance of a woman who returns shortly after her husband was killed.