Elwood P. Dowd is an endlessly pleasant & delightfully eccentric bachelor living in a small town that isn't quite aware that its newest citizen is a 6'3" white rabbit named "Harvey," that only certain people can see. After supposedly meeting this rabbit - its origins attributed to the Celtic legend of the Pooka - Dowd's sanity is put into question by his equally eccentric sister.
Elwood casually drives her guests from their house by introducing and carrying on one-sided conversations with his invisible and silent friend. His sister and her daughter, (Elwood's Niece) resort to taking him to the local sanitarium to have him committed. However, owing to people not paying attention, interrupting, and cutting Elwood off as he is about to introduce his imaginary friend, it doesn't imediately appear that there is anything wrong with him, although the examining doctor is sure he is doing the right thing by admitting him..
Elwood is carted upstairs by a rough-handed and simple-minded man in the white coat while the examining doctor ushers the sister into the head psychiatrist's --Dr. Chumley's-- office to give a description of the problem. As Elwood is "escorted" to hydrotherapy, he tells the man in the white coat his friend "Harvey" is a "Pooka" The Aide later looks up the definition in the dictionary: "Celtic mythology, a miscihevous spirit that takes animal form and appears sometimes to some people for the purpose of doing this and that."
As the sister, still highly upset over Elwood driving off her friends, bemoans Elwoods delusions to Dr. Chumley, her frazzled manner and insistence that Elwood actually does have a six foot 3 inch invisible rabbit for a friend convinces Chumley it is she, not Elwood, who is hallucinating.
He quickly has the sister carted upstairs and, fearing a lawsuit for incarcerating and treating a sane man, brings Elwood down and makes every manner of friendly gesture. He fires the examining doctor. The nurse, who is actually quite fond of him, is crestfallen; but, upset at being fired, he is unaware of her feelings for him, and this angers her.
Elwood in his normal good natured way takes no exception to any of the events and once again, as he is about to introduce "Harvey," gets cut off, interrupted and ignored while the Chumley pursues his patronizing commentary. Never one to interrupt, the pinnacle of politeness, Elwood lets him say whatever he is going to say and is finally given a pass to leave.Soon the sister's friends, one of them a Judge, and her daughter, and another friend, show up and the mistake is uncovered and the entire group goes into a panic trying to find Elwood. The sister and the Judge promise to sue the sanitarium for wrongful incarceration and rough treatment.
Elwood appears to drink quite a lot and retires to his favorite watering hole, Charlie's. Dr. Chumley himself tracks him down there, by himself, but while he is doing this, everyone else is looking in other places and doing other things. When it finally becomes apparent to everyone that the Doctor has been gone for over four hours and that Elwood is not back in custody, another major panic ensues and the group descends on Charlie's to see what has happened to the Dr. Chumley.
Elwood, of course, is sitting alone, drinking, and maintains that the Doctor left the bar with Harvey. They are convinced he is a madman and has done away with the doctor. However, in his easygoing and pleasant manner--a somewhat slowed down version of Bugs Bunny, Elwood sidetracks everyone with drinks and conversation and disarms them.
Eventually the subject does turn back to the missing doctor, and the panic ensues again.
The police are summoned, the heavy handed man in the white coat muscles Elwood back to the sanitarium, but not before becoming infatuated with the sister's daughter, (Elwood's niece) and making advances toward her. Although he is a galoot, she is attracted to him. The entire entourage, including the daughter from the house, the fired doctor, the nurse from the sanitarium, the guard, the judge, the sister, and the police, arrive at the sanitarium in a police car and a taxi to discover the doctor is there.
Dr. Chumley indeed did leave with Harvey, the Pooka, but he wisely does not say anything about it to anyone, dismisses them all and tells everyone that everything is under control. He tells the doctor he fired that he is a fine doctor and can have his job back and disappears into his office. The reinstated doctor then decides to administer a very powerful injection into Elwood that will make him cease with his delusions about the rabbit for once and for all.
Meanwhile the cab driver wants his money for the fare. No one seems to have any money so the sister say she'll pay. Digging everything out of her handbag, she discovers it is gone. She assures the driver that if he waits until Elwood has his injection, she'll pay him handsomely when they are driven back home. The Cab driver however has dour insights into the effects of the injection about to be administred to Elwood. He says he's given many people a ride to the sanitarium for that injection and it changes them into crabby mean people: normal but irritable and unpleasant. Elwood, the most disarming and engaging and mild mannered man in the universe, is on the verge of being turned permanently into a real nuisance, so the sister abruptly changes her mind and rushes in and prevents the injection.
Chumley then asks to see Elwood, and requests that Elwood allow Harvey to stay with him for a while and help him out by making his long-needed sabbatical a reality, because apparently this is one of the things Harvey can do if he is so inlcined. Through all of this, no one ever sees Harvey, Harvey never speaks a word, and only exists through the opinions and announcements of the only two people who can perceive him: Dr. Chumley and Elwood P. Dowd. Elwood asks Harvey if he will work with Chumley and tells Cumley Harvey agrees to, and he leaves without him.
The re-hired Doctor pays for the cab, he warms up to the nurse who has a crush on him, the entourage leaves, with the heavy-handed guard and the maid making a date to see each other, and they all go to get in the taxi.
As the sister digs in her purse for something she discovers her money purse. She looks over her shoulder and says knowingly, "Harvey," and shakes her head. She knew Harvey existed, and was michievous, and her only reason for wanting to have Elwood committed was that Elwood's insistence on introducing him to her guests was driving everyone away.
Harvey wasn't imagined at all. The camera validates Harvey's existence as Elwood, outside the security gate, looks back to "see" Harvey-- and an invisible hand turns the crank to open the gate to let Harvey walk out.
He is a Pooka, a mischievous spirit in animal form, invisible to most, who had engineered the whole fiasco to begin with--and somehow, when things were on the brink of complete chaos and hopelessness, he would manipulate the entourage and players into another scene of hilarious melodrama.
It ends with the doctor and nurse falling for each other, the white coat and the daughter falling for each other, the Pooka "changing his mind" about being with Dr. Chumley and returning to Elwood, and the situation between the sister and Elwood unhcanged-- and a bigwig Psychiatrist who now has an invisble friend he is afraid to tell anyone about.