Peter Sellers stars as Harold Fine, a self-described square--a 35-year-old Los Angeles Lawyer who is not looking forward to middle age and his upcoming wedding. His life changes, however, when he falls in love with Nancy, a free-spirited, innocent, and beautiful young hippie. After Harold and his family enjoy some of her "groovy" brownies, he decides to "drop out" with her and become a hippie too. But can he return to his old life when he discovers that the hippie lifestyle is just a little too independent and irresponsible for his tastes?

Johnny Barrett, an ambitious journalist, is determined to win a Pulitzer Prize by solving a murder committed in a lunatic asylum and witnessed only by three inmates, from whom the police have been unable to extract the information. With the connivance of a psychiatrist, and the reluctant help of his girlfriend, he succeeds in having himself declared insane and sent to the asylum. There he slowly tracks down and interviews the witnesses - but things are stranger than they seem ...