Mr. Peabody is no ordinary dog; he's a highly intelligent, talking canine who has dedicated his life to science and innovation after facing rejection for adoption in his puppy days. When he comes across an abandoned infant named Sherman, he legally adopts the boy, raising him with love and guidance using a fantastic time machine known as the WABAC. Their lives together become a whirlwind of exciting adventures through history, enriching both their lives.
However, as Sherman grows up, he must face the realities of school, which brings about a poignant challenge for Mr. Peabody. On Sherman's first day, things take a turn for the worse when he inadvertently impresses his classmate, Penny Peterson, with his historical knowledge. This leads to Penny bullying him and cruelly calling him a "dog" because of his unique upbringing, which eventually triggers Sherman to bite her in frustration. Their teacher, Mr. Purdy, and the prejudiced children's services agent, Ms. Grunion, create a precarious situation, threatening to take Sherman away if their home is deemed unfit during an upcoming inspection.
In a bid to resolve their issues, Mr. Peabody invites Penny's parents, Paul and Patty, over. While Peabody puts on a charm offensive to win over her parents, Sherman finds himself begrudgingly entertaining the troublesome Penny. In a moment of rebellion, he shows her the WABAC, igniting a sequence of events where they joyride through time. When Penny finds herself stuck in Ancient Egypt, engaged to the young Pharaoh King Tutankhamun, the duo must embark on a daring adventure to rescue her from a disastrous fate, even while facing the arrogance of Egyptian royalty.
Their travels take them to Renaissance Florence, where they seek help from the great Leonardo Da Vinci to recharge the WABAC. As they partake in yet another escapade, exciting emotions bloom between Sherman and Penny, culminating in moments of mischief and tension. However, their journey takes a dark turn when they accidentally navigate the WABAC into a black hole, threatening their very existence.
With their predicament forcing them into the Trojan War, Sherman’s rebellious spirit leads him to join King Agamemnon's forces, prompting Mr. Peabody and Penny to charge in and retrieve him before calamity strikes. Chaos erupts when their adventure culminates in the destruction of the legendary Trojan Horse, leading to Peabody’s apparent demise in a climactic showdown.
Desperate to rescue his father, Sherman devises a plan to return to the past moments before Mr. Peabody’s fall. Unfortunately, complications with paradoxes arise, resulting in the chaotic merging of timelines as dual versions of Peabody and Sherman confuse the already tense situation. As Ms. Grunion seizes the opportunity to claim Sherman, he finds himself torn between his two worlds, forcing Mr. Peabody into an instinctive reaction born from desperation.
Amidst mounting chaos, they flee in the WABAC, confronting a time vortex that threatens to swallow everything as various historical figures are dragged into their present. In a heartwarming moment, Sherman proclaims his desire to be as good as Mr. Peabody, prompting the support of not just historical figures, but even U.S. Presidents like George Washington, Abraham Lincoln, and Bill Clinton, rallying behind Peabody’s custody rights.
In a show of unity, Sherman proposes a return to the future to amend their mess, successfully restoring order by returning everything to its rightful time. At the conclusion of their wild adventure, Mr. Peabody and Sherman emerge strengthened, ready to embrace life now closer than ever, complemented by their newfound bond with Penny, who has become a close friend.
