Sur plus de vingt ans et d'un continent à l'autre, Mary et Max raconte l'histoire d'une relation épistolaire entre deux personnes très différentes : Mary Dinkle, une fillette de 8 ans joufflue et solitaire, vivant dans la banlieue de Melbourne, en Australie, et Max Horowitz, un juif obèse de 44 ans, atteint du syndrome d'Asperger et habitant dans la jungle urbaine de New York.
A tale of friendship between two unlikely pen pals: Mary, a lonely, eight-year-old girl living in the suburbs of Melbourne, and Max, a forty-four-year old, severely obese man living in New York.In the mid-1970's, a homely, friendless Australian girl of 8 picks a name out of a Manhattan phone book and writes to him; she includes a chocolate bar. She's Mary Dinkle, the only child of an alcoholic mother and a distracted father. He's Max Horowitz, living alone in New York, overweight, subject to anxiety attacks. He writes back, with chocolate. Thus begins a 20-year correspondence, interrupted by a stay in an asylum and a few misunderstandings. Mary falls in love with a neighbor, saves money to have a birthmark removed and deals with loss. Max has a friendship with a neighbor, tries to control his weight, and finally gets the dream job. Will the two ever meet face to face?