Dr. Leo Marvin, an egotistical psychotherapist in New York City, is looking forward to his upcoming appearance on a "Good Morning America" telecast, during which he plans to brag about "Baby Steps," his new book about emotional disorder theories in which he details his philosophy of treating patients and their phobias. Meanwhile, Bob Wiley is a recluse who is so afraid to leave his own apartment that he has to talk himself out the door. When he is pawned off on Leo by a psychotherapist colleague, he becomes attached to him. Leo finds Bob extremely annoying. When Leo accompanies his wife, Faye, his daughter, Anna, and his son, Sigmund, to a peaceful New Hampshire lakeside cottage for a month-long vacation, he thinks he's been freed from Bob. Leo expects to mesmerize his family with his prowess as a brilliant husband and remarkable father who knows all there is to know about instructing Faye and raising Anna and Sigmund. But Bob isn't going to let him enjoy a quiet summer by the lake. By cleverly tricking the telephone operator at Leo's exchange, Bob discovers the whereabouts of him and his family. Despite his phobia about traveling alone, Bob somehow manages to talk himself onto a bus, and he arrives in New Hampshire. Leo's vacation comes to a screeching halt the moment he sees him. With his witty personality, his ability to manipulate people, and his good sense of humor, he quickly becomes an annoyance to Leo, but not to Faye, Anna, and Sigmund, because they think he is fun while Leo is dull. Fearing that he's losing his family to him, Leo frantically tries to find a way to make him go back to New York City, and it's not as easy as he had hoped. He finds himself stepping outside the law to try to get Bob to stay away from Faye, Anna, and Sigmund--he slowly goes berserk, and makes plans to kill Bob.