In the vast reaches of space, Oil Can Harry is chasing Queen Pearl Pureheart of the Interstellar Federation, when she informs him he is both speeding and has violated sector code 6655A, attacking a star cruiser.

She then summons Mighty Mouse to her rescue with the "Save Our Necklace", prompting Harry to release his "Catatomic-catom-aton", a giant robot cat that can destroy planets, melt steel walls, and crush spaceships.

The Save Our Necklace sends a 3D holographic distress signal that finds Mighty Mouse no matter where he is, and he sees it while lifting weights at his house, which is in the shape of a crescent moon and made of cheese.

While the cat robot battles Mighty Mouse unsuccessfully, Pearl decides she can't wait, so she sneaks out and pulls out the catship's whisker, freeing it, but leaving her behind.

The first mate uses the necklace to contact Mighty Mouse, who then discovers the cat robot was just a giant wind-up toy, and unwinds it.

This is because he tried to force her to tell him where the ISF hides the Domesday ("dooms day") Device, a giant vacuum cleaner that sucks up everything in the galaxy, and she refused.

", when the security scanners detected Mighty Mouse, and the rest of the cat crew verbalizes warnings since the alarms failed.

The mouse ship then continues its mission, and Harry hides a cassette tape player to fool the crew into believing Pearl is sound asleep.

Upon arriving at Humongo, Mighty Mouse lands the ship at the repair facilities, while Harry takes pictures of the secret installations with a camera.

He lets the fake Pearl know he's going to the gym for a workout, after pointing out that "she" could have just bought the postcard set instead of taking all the pictures.

Harry even reveals that he found the location of the Domesday Device, then notices the necklace, wondering what it is, but doesn't care in the end.

However, the crew is too late; they catch Mighty Mouse in the act of switching the brains back, by running in a wheel that revs up enough energy, and commanding Pearl to push the button at the right moment.

Pearl, the King, and the Humongo Police Force arrive at the vault and see Mighty Mouse inside in, and they jump to conclusions.

Pearl subsequently arrests Mighty Mouse on behalf of the ISF, and he goes peacefully, stating he would be the absolute last one to resist the authority of law and order.

King Muss rules Mighty Mouse not guilty and dismisses the case, but Harry intercepts the Device and replaces it with a time-ticking booby trap set to explode.

After getting the package containing the Domesday Device open, Harry complains about the fact it's a kit, requiring two D-cell batteries, so he and his crew have to put it together themselves.

Harry then "votes for himself" while using a laser to destroy a national monument, and the mice shiver in fear, proceeding to repair the ship.

Harry and Swifty rush back to the ship and take off for their secret lair, and off-screen Mighty Mouse makes things right with the mice.

Pearl and her crew take off while Mighty Mouse gives the mice a crash course in agriculture, powering a machine by forcing the same Eyesaurus to run on a treadmill for hours on end.

Harry then employs his friend Matta Furri, the greatest spy in the universe, to lure Mighty Mouse into a trap by getting romantic with him.

The last ounce of hipporanium is included as a sample in Mr. Whiz Bang's Super Deluxe Chemistry Set, which the toymaker under pressure gives Harry.

Mighty Mouse saves them by creating a picnic, which the ants can't resist, and Pearl reveals that since they always carry a spare, she rebuilds the ship by stomping on a buzzer.

He gives the mice only five seconds to surrender, but when Pearl calls to tell him that nobody will obey him, he makes good of his threat to use the Domesday Device to suck up everything and starts to do so.

Pearl and Mighty Mouse watch a special news bulletin, learning that Harry is doing this, when the Device sucks up the anchor giving the report.

Mighty Mouse gets the catship to open its "mouth" and sticks the nozzle into the port, making it too windy for Harry and Swifty to move.

Later, a parade is held to celebrate the victory, and the Device's nozzle is tied in a knot, forever trapping the cats inside, leaving the aforementioned dog squad to pursue them.