The Grinch wakes up in a good mood one morning until his reflection in the mirror (possibly an envisioning of his father) speaks to him, prompting him to repeat the "Grinch's Oath", reminding him of his evil, venomous nature, and he leaves to prove himself. Meanwhile, The Cat in the Hat goes on a picnic. Their paths cross when the Grinch bumps his car into the Cat's, and things quickly escalate into a fierce car chase after the Cat inadvertently insults the Grinch by calling him "Mr. Greenface".
The Cat returns to the safety of his home, but the Grinch follows him to demonstrate a device he has invented, an "Acoustical Anti-Audial Bleeper", also referred to as a "Vacusound Sweeper", that scrambles all sounds within a 50-mile radius, including the Cat's voice. Back home, the Grinch decides to upgrade the sweeper into a "darkhouse", an anti-lighthouse that spreads beams of darkness.
The Cat becomes livid with the Grinch's hijinks and has a psychiatric session with him in a thought bubble to find out what makes him so mean-spirited. He gets nowhere with the imaginary Grinch (though he is briefly triggered by the memory of his deceased mother), so the Cat then decides to go over and have a talk with him. The Grinch makes it so dark that the Cat can't see where he's going, and he crashes his car when he passes a "Dead End" sign. The Grinch decides to liven things up by changing the beam of darkness to persimmon pink.
The Cat takes refuge in a nearby restaurant, while the Grinch sends beams that make things change colors, and literally and crazily come to life, and his hijinks result in confusion all over the restaurant. The Cat has now become sick and tired of the Grinch and ponders to himself how he can change the Grinch, eventually finding an idea and rallies up everybody in the restaurant to follow him to the Grinch's house. There, he leads everyone in a song to remind the Grinch of all of the love he received from his mother and implore him to change his ways and be a better person. Before the Grinch can get to the darkhouse to scramble it, he collapses in grief over the memory of his mother (whose reflection in the puddle of his tears comforts him) and he and Max dismantle the darkhouse.
The next morning, the Grinch is again happy, but when the reflection tries to turn him evil again, Max reveals he left the Vacusound Sweeper intact and scrambles the reflection's words.
Both the Grinch and the Cat in the Hat were recast with different voice actors than the ones used in previous specials, all of whom had died. Bob Holt voiced the Grinch (Hans Conried, who voiced the Grinch in Halloween Is Grinch Night, died four months before this special had aired, while Boris Karloff, the original voice of the Grinch, died in 1969) and Mason Adams took over voicing the Cat in the Hat from Allan Sherman, who died in 1973.
The special was first released on VHS in the mid-80s via CBS/Fox Video's Playhouse Video division, and reissued later in the 90s by Random House Home Video. The Random House Home Video release used the title The Cat in the Hat Gets Grinched. The special retained its normal name on VHS re-releases (including Dr. Seuss Sing-Along Classics by 20th Century Fox Home Entertainment with CBS Video and Fox Kids Video). It was also re-released on VHS in 2000 by Paramount Home Entertainment.
