Send in the Clown
Mario and the Princess are enjoying a game of tennis, using fireballs are the ball, and Yoshi is taught the harsh reality of being a ballboy: They don't eat the balls. "You call this fun!?" Yoshi remarks. Not even far enough to be out of sight, Kootie Pie and Big Mouth are placing posters for their circus around the place while Bowser flies around in the Clown Car telling people to go. We're not even two minutes in, and he reveals to the audience that he plans to feed the cave people to his ravenous dinosaurs. So much for suspense.
After Luigi shows his immeasurable support for his brother and Yoshi eats the ball because he still doesn't understand the rules, they realise that the place is deserted and that King Koopa's up to no good, so after going to Star World for no more than ten seconds and arriving at Bowser's castle, they follow the cave people inside and find a big ol' circus, which is apparently a surprise even though they quite clearly saw the poster.
After Yoshi compares inevitable death to tennis, they take a scene and watch the events, which consists of little more than a Mechakoopa riding a bicycle on a tight rope and Chargin' Chucks playing baseball on those swinging bar things, although some dinosaur clowns are brought in to cock up the other acts and be dicks towards Big Mouth. Even when pies are thrown at the audience, Mario suspects no evil intentions, but once the clowns drop their hats and growl, everyone suddenly realises it's a trap.
Big Mouth opens a trap door and turns the seats into slides, all of the audience but Mario are dropped into the cage, while our plumber hero is made to do a dinosaur taming act, which just consists of Mario insulting their breath and then suggesting they say grace before munching on him, until Peach throws him a Fire Flower, allowing him to merely explode the dinosaurs. He busts the lock, and tells the others he'll be back once he finds a feather.
Awkward song time! In the time it takes for the song to finish, Mario kills about a dozen enemies, gets a feather, does some crazy platforming, beats up Big Mouth and Bowser repeatedly, and then destroys the entire circus. Quite a feat, especially considering that last one only required him to pull out a stake.
Before he can release his buddies, Bowser tells him he can't if he wants Peach back, and starts throw giant bowling balls and Mechakoopas at him, before making the Clown Car try to crush him. All it takes is one Mechakoopa to be thrown in his Clown Car, and that's enough for it to tip the passengers out, which apparently counts as beating King Koopa. So the Princess is saved, the prisoners are free and a big celebration is planned on their return!
Another circus performance. With ice cream, tight rope walking (not even bicycle riding), juggling and a clown who can't keep his balance. It still gets plenty of applause, so they must've done something good. Or the cave people have low standards.
Every single reference I've seen to this episode, whether it be the DVD packaging, episode guides or anything, always seem to call it Send in the Clowns, even though the episode itself uses the singular clown. Proof!
Kootie Pie does absolutely nothing in the episode. She puts up some posters, laughs at Big Mouth's mistreatment, helps her dad pull away some curtains, and is then crushed by the collapsing circus tent. Aside from her laughing and scream, she has no dialogue. How very odd.
Luigi has apparently eaten Mario before.