In the standard episodes Top Cat either thinks of some zany scheme or some zany scheme ends up happening, Officer Dribble finds out about their plan, tries to stop T.C and his gang, and the episode ends with both sides victorious. Dribble has stopped crime, and the alley cats are back in their alley.
I mention this because the film starts off as though it were little more than a normal episode. In such an episode, Top Cat wants to impress a girl, hears that the Maharajah of Pookajee is in town (again), and plans to "kindly alleviate the pressure" of the Maharajah's excessive fortune. They swindle their way in by taking the tickets from some obnoxious jerk, divert Officer Dribble's attention, stage a bet they are sure to win, get chased by Dribble, and come out of it with some type of nice reward such as a measly couple thousand dollars.
In the first act of the film, such events occur almost exactly. Top Cat hasn't had a girlfriend since Jazz the cat came into town and tried to take T.C.'s place (episode 3), which is why Top Cat was able to try and court the nurse Miss La Rue
(episode 7); this means that the premise wouldn't be unfamiliar to fans. The Maharajah would be the most direct source for some fortune (episode 2), and a Laszlo-Laszlo concert would be the perfect reason for the Maharajah to be in town (episode 5).
Ended there, no one would bat an eye. Of course, if it ended there then this wouldn't be a movie. Instead, the jerk who T.C. swindled to meet the Maharajah turns out to be a power-hungry Big Bad with an inflated ego and the love interest is his Dragon. And that the reward T.C. obtains isn't either money or jewels acts as a nice foreshadow while also demonstrating T.C.'s integrity (albeit sad about it, he was willing to leave without his efforts paying off and then kept his reward safe when he could have thrown it away).
You can then factor in that multiple characters from the series reappeared: Big Gus, Griswald, and Benny's horse. You also have the inclusion of a few plot elements and jokes from the series (a thief disguised as one of the crew, Dibble's quest to be chief, Benny getting zoo animals, saving the old lady, etc).
If you had to create a 90-minute finale to Top Cat, you would be hard-pressed to do better. Except for the 2D-on-3D animation style; the gorilla could probably do better.