John Grogan holds fond memories of his childhood pet, a mutt dog named Shaun. John and his newlywed wife, Jenny, are contemplating having children and decide that a dog will be good parenting practice. The couple visits a local breeder specializing in Labrador retrievers where Jenny picks out a rambunctious, gangly puppy. The puppy is named Marley after Bob Marley, whose music welcomed John and Jenny to South Florida. Marley proves to be nothing like Shaun. He is a salivating, headstrong, damage-effecting, barreling-through-life flash of yellow fur. Marley is kicked out of obedience school and almost barred from the Grogan household, but in the end turns out to be an excellent teacher of love and loyalty. Marley is the world's best worst dog.

John and Jenny's life with Marley soon becomes a comfortable routine. Jenny immediately falls into a nurturing-mother mode and meticulously combs Marley for any sign of fleas. She comes home from work on her lunch hour in order to feed, water and exercise him. John takes Marley for a walk every morning, and the entire family spends the evening playing in the backyard or walking the nearby beach trails. Their happiness is not without mishaps, however. Marley is terrified of thunderstorms. He turns the garage into a bloody battlefield every time a storm hits as he tries to escape the noise. Tranquilizers help mildly, but for his entire life Marley shudders at the first sign of thunder and lightning. Marley also loves to eat things including, overripe mangoes, delicate gold necklaces, shoes, pillows and kitty litter.

For all his shortcomings, Marley proves himself truly loyal and completely devoted to the Grogan's. When Jenny suffers a miscarriage, Marley sits with his head in her lap as she sobs into his fur. During Jenny's second viable pregnancy, she is put on bed rest. Marley becomes her constant companion, bringing her toys and sleeping next to her side of the bed. Marley senses that Jenny needs him to be strong for her. His usually flamboyant demeanor becomes respectful in the face of Jenny's physical and emotional needs. Even when Jenny is suffering from postpartum depression and pummels Marley with her fists after he destroys a couch cushion, Marley sits patiently, allowing his mistress to vent her frustration. Marley's ability to sense what his family needs and rise to the occasion is a testament to his love for the Grogans. John realizes as Marley approaches old age that the wild puppy has really become a wise teacher. Simply by being Marley, he has brought John and Jenny closer together as a couple, provided protection and friendship to the entire family and shown everyone who ever met him that life is met to be lived to its fullest, even if it means replacing several door screens and buying new furniture here and there.

Marley's hysterical and heartwarming story touches the reader deeply. It is impossible not to remember one's own treasured pets and the lessons that they taught. The reader cannot help falling in love with this rascally dog, who brings vitality to all who know of him. By the end of the book, the reader is almost wishing that s/he, too, had a Marley of her/his own and is saddened by the Grogan's loss when Marley dies. Marley came into the Grogan household to teach John and Jenny how to parent, but he ended up teaching them how to live.