The good-natured Elwood P. Dowd (James Stewart) has a constant companion Harvey, a six-foot tall rabbit that only he can see. To his sister, Veta Louise (Josephine Hull), Elwood's obsession with Harvey has been a thorn in the side of her plans to marry off her daughter. But when Veta Louise decides to put Elwood in a mental hospital, a hilarious mix-up occurs and she finds herself committed instead. It's up to Elwood to straighten out the mess with his kindly philosophy, and his 'imaginary' friend.