Soon after tying the knot, John Grogan and Jenny Grogan decide to escape the harsh Michigan winters and make a move to sunny South Florida. They land jobs as reporters at rival newspapers, with Jenny quickly receiving major front-page assignments at The Palm Beach Post. In contrast, John finds himself stuck writing obituaries and two-paragraph pieces covering mundane happenings like a fire at the local dump.
As John senses Jenny's growing interest in motherhood, his friend and co-worker Sebastian Tunney suggests they adopt a dog to test their readiness for family life. They choose a lovable yellow Labrador from a litter, naming him Marley, after the famous reggae artist Bob Marley. Initially priced at $200, Marley becomes an instant handful, proving to be far more mischievous than they anticipated.
When Jenny is called away for work, John is left to manage the energetic puppy. Their home quickly turns into a chaotic mess as Marley refuses to stay in the garage and wreaks havoc on their furniture. Within one night, he destroys their pillows and mattress, and when left alone, he decimates the house further. As he grows larger and more uncontrollable, John and Jenny are forced to enroll him in obedience training, led by Ms. Kornblut. Unfortunately, Marley’s antics lead to his expulsion from the class after he causes a ruckus.
With Marley’s chaos inspiring him, John’s editor, Arnie Klein, offers him the opportunity to write a column about his experiences. Initially struggling for content, John realizes Marley’s daily misadventures provide ample material, which quickly resonates with readers and boosts the newspaper's popularity. However, Jenny faces emotional turmoil after a heartbreaking miscarriage early in her pregnancy, finding solace in Marley’s companionship.
A trip to Ireland for their delayed honeymoon results in more mischief, as their dog causes mayhem in their absence, leading the young caretaker to label Marley as "evil incarnate." Once back, Jenny receives joyous news of her second pregnancy, resulting in the birth of their son, Patrick, who forms an immediate bond with Marley. After the arrival of their second child, Connor, Jenny chooses to become a stay-at-home mom, while John takes on a daily column to support the growing family.
Concerned about safety following a local crime, they opt to move to a larger home in Boca Raton, where Marley enjoys the freedom to swim in the pool. Despite denying it, Jenny struggles with postpartum depression, which manifests in her growing impatience with Marley and John, prompting her to consider giving him away. John, however, views Marley as a beloved family member and resists this idea.
Ultimately, Jenny comes to see Marley's significance in their lives, leading to a reaffirmed commitment to keep him. Life continues with ups and downs, including a career shift for John as he takes a reporting position in Pennsylvania, yet he soon realizes that he prefers writing columns.
As the years pass, Marley’s age catches up with him, showing signs of arthritis and deafness. After a critical health scare, it becomes clear that Marley's time is limited. In a heart-wrenching farewell, John remains by Marley’s side as they say goodbye, honoring their cherished pet with a final resting place beneath a tree in their yard.
