Identical twins separated at birth: One a prominent psychiatrist; the other a life long mental patient. When the doctor gets called to his brother's institution, fate intervenes and the brothers swap places.
Insanity is relative
Did You Know?
When the patients are playing in the "band", the drummer's action don't match the sound. In the closer shots, you can see that he is not even actually hitting the drum heads with the sticks. See more
I'm not Fred! I'm Ted!
References The Parent Trap