After getting married, John and Jenny Grogan move from Michigan to southern Florida to escape the harsh winters. They both get jobs as reporters, but at different newspapers. Jenny works at The Palm Beach Post and is given important assignments that make it to the front page. On the other hand, John ends up at the South Florida Sun-Sentinel where he writes obituaries and short articles about uninteresting news like a fire at the local garbage dump.
When John feels that Jenny is considering becoming a mother, his colleague Sebastian Tunney proposes that they adopt a dog as a test to determine if they are prepared to raise a family. They choose Marley, a yellow Labrador Retriever from a litter of newborn puppies. They name him after the famous reggae singer, Bob Marley. However, Marley quickly proves to be uncontrollable. They decide to bring him to Ms. Kornblut (played by Kathleen Turner), who strongly believes that any dog can be trained. Unfortunately, Marley refuses to follow commands and is ultimately expelled from her training class.
Editor Arnie Klein offers John a biweekly column in which he can explore the enjoyable and comical aspects of everyday life. Initially struggling to decide on a subject, John eventually recognizes that the antics of Marley could be an ideal topic for his debut article. Arnie approves of the idea, and John comfortably adjusts to his newfound role.
Despite causing chaos in the household, Marley’s mischievous behavior provides ample content for John’s popular column, which helps increase the newspaper’s circulation. Unfortunately, Jenny suffers a miscarriage during her first trimester of pregnancy. To compensate for their delayed honeymoon, John and Jenny take a trip to Ireland while someone else takes care of Marley. However, managing Marley becomes challenging for this person, especially during thunderstorms. When they return, Jenny discovers she is pregnant again and successfully gives birth to a healthy baby boy named Patrick. Later on, she has another son called Connor and decides to quit her job and become a stay-at-home mom. The couple also moves to a bigger house in a safer neighborhood in Boca Raton where Marley thoroughly enjoys swimming in the backyard pool.
John and Jenny have recently welcomed their daughter Colleen into their family. However, Jenny, although she denies it, displays symptoms of postpartum depression. This is noticeable in her growing impatience towards Marley and John’s suggestion to find a new home for the dog at Jenny’s request. Nevertheless, Jenny eventually recognizes that Marley has become an indispensable member of their family and ultimately consents to letting him remain.
John celebrates his 40th birthday and decides to change his job. He accepts a position as a reporter with The Philadelphia Inquirer, and with Jenny’s blessing, the family moves to a farm in rural Pennsylvania. Life is perfect until Marley, their aging dog, starts showing signs of arthritis and deafness. Despite surviving a near-fatal attack of gastric dilatation volvulus, Marley’s health deteriorates. The family realizes that surgery is no longer an option and decides to euthanize Marley. John stays by Marley’s side until the end. The family holds a final farewell for their beloved pet and buries him under a tree in their front yard.
