Conrad and Sally Walden, siblings living with their single mother, Joan, find themselves in a whirlwind of chaos when she's called to work at the real estate firm run by the uptight Hank Humberfloob. While Joan is out, strict orders set forth to keep the living room off-limits for an upcoming office party soon dissolve into uproar once babysitter Mrs. Kwan dozes off.
In a surprising twist, the mischievous Cat in the Hat, a charmingly chaotic character with a signature red-and-white striped top hat and a bold red bow tie, appears to turn their mundane day into a delightful escapade. Fuelled by a desire to show them the essence of fun, the Cat creates a spectacular mess in their home while revealing two rambunctious pals, Thing 1 and Thing 2, who he frees from a forbidden crate that he uses as a portal to his world.
Despite the Cat's warnings to stay away, Conrad's curiosity gets the better of him. He unlocks the crate, inadvertently causing the family dog, Nevins, to dash out uncontrollably. This mishap leads the trio on a wild chase in the Cat's fantastically powered car to retrieve their beloved pet. Meanwhile, the children’s next-door neighbor, Larry Quinn, a dishonest and slovenly man, is exposed as a con artist aiming to marry Joan for her wealth and remove Conrad from the picture by sending him off to military school.
As the plot progresses, Larry kidnaps Nevins, prompting the Cat to outsmart him and retrieve the dog. When Larry tries to inform Joan about the Cat's uproarious antics, he is intercepted by Thing 1 and Thing 2, who masquerade as police officers. This hilarious distraction leads to Larry directing Joan back to the house, unaware of the chaos that awaits them.
Upon their return, Larry’s allergic reaction to the Cat triggers a comical series of events as he confronts a transformed living room, now dubbed “The Mother of All Messes,” complete with gooey traps. The adventure escalates as the Cat, along with Sally and Conrad, takes on the absurdity to find the crate and restore order. Ultimately, after a passionate disagreement, the children order the Cat to leave, but not before he astounds them by using an ingenious cleaning device to set things right.
Just as Joan arrives home, Larry, covered in goo, mistakenly believes he has caught the children in their mischief. However, Joan, upon witnessing the spotless home, disbelieves his outlandish claims and dismisses him in utter humiliation. With the party a resounding success, Joan delights in her time spent with her children, leaping joyously on the couch while the Cat and his whimsical companions bid farewell, waddling off into the sunset.
