A man, going along the street, suddenly loses his reason, and when the astonished crowd realizes the true state of affairs a policeman is called, who humors the queer acting fellow, and finally, through strategy, coaxes him into a sanitarium, where he is attended by a brain specialist. The latter hypnotizes the fellow, and, after removing his diseased brain, replaces it with a monkey's thinking piece. Immediately there is a great transformation, and the patient turns into a monkey and begins cutting up all the antics for which this animal is noted. The doctor takes his charge on a leash and starts out, and as they are going down the street the mischievous fellow plays all sorts of tricks, upsetting stands in the market place and causing no end of excitement and amusement to the general public. Finally they wind up at a dime museum, where Mr. Monkey is put on exhibition, and before long the standing room sign is put up in order to keep out the clamoring crowds that flock to see the latest acquisition.