Luigi, apparently given the role of Yoshi's mother, is putting him to bed and cracks a nonsensical gag before telling him his favourite bedtime story about how the two of them first met. Cue flashback!
Luigi explains that he, Mario and the Princess came to Dinosaur World for a vacation after Bowser was banished from the Mushroom Kingdom, only to find the Dome City deserted, except for Koopa football players who kidnap Peach, the creepiest Magnum Bill ever, a Magikoopa, and a "Fire Sumo", the last of which causes an earthquake that Luigi falls into.
After making the most awesome scream ever, he happens to land on a magic balloon and stay airborne for all of two seconds before reverting to normal and continuing to fall towards imminent death via lava. Ever conveniently, he lands on a skull raft ("Skulls? Yuk! Skulls mean bad medicine!" Yoshi adds as a moral lesson), which ever inconveniently is heading towards a lava fall, but twice conveniently, he lands on solid ground again! Yoshi then acts like a member of a Monty Python stage show and calls out what happens next, where Luigi starts bashing blocks.
Out comes a Yoshi egg which immediately hatches, and the first words he utters are "mama?"
A stark contrast to his first words in the game, to say the least.
After Yoshi finds this knee-slappingly hilarious, inconvenience continues striking by two giant dinosaurs strolling in and feeling the need to munch down on the two of them. He feels the need to threaten them first before escaping across the back of a Blaargh and down a pipe, which leads into the ocean and causes them to get chased by a fish, but it's barely bigger than Yoshi and has a tiny mouth, so even when there are Urchins swimming at them as well, it seems like a half-hearted way to make-up for the five seconds of chasing we got from the dinosaurs.
After the fish falls asleep, another comes straight at them, only to be easily exploded, and the chase from the Torpedo Ted barely lasts before they're rescued by dolphins. After being brought to shore, Yoshi wanders off while Luigi dries his cap and shows off his funky hairdo, and is found poking a flower while a Wiggler crawls up behind; however, in the time it takes for Luigi to scold the disobeying dinosaur, several others gang up on them.
Yeah, he could easily just run in the opposite direction, but if he did that we couldn't get to hear another of his awesome screams.
Yoshi, apparently not worried about being eaten alive by five hungry caterpillars, demonstrates how he's hungrier than them by gobbling them whole. Luigi cracks another gag at his brother's expense while the Wigglers make no attempt at running away or attacking, and are soon all disposed of. Mario then appears out of nowhere and believes Yoshi is the threat, although it's only after he's eaten by him that they're told they're all allies. Mario explains that the Princess is locked in Bowser's castle, although he managed to bust out, so they need to go back and rescue her. Yoshi coming along with them, naturally.
They arrive at the castle and enter via a pipe, and even though things are clear as day, they aim to bash the light switch block, and when they do (after leaving Yoshi unattended nearby), it only dims the place and adds some spotlights, but sure, whatever moves the plot along. Naturally, Yoshi has wondered off again, which leads us into this episode's awkward song time! In a nutshell, it just shows that Mario and Luigi are total dunces and pale in comparison to Yoshi's mad skillz, but they manage to reach Bowser's throne room.
While Yoshi eats everything, the brothers look around and eventually come across Bowser with an army of Mecha Koopas, but nothing comes of it as they run back into the throne room, where Yoshi has eaten absolutely everything, upsetting Bowser to such a great degree that he repeats his command of getting the intruders to his Mecha Koopas. Yoshi merely eats them all, frustrating King Koopa even more, but he has one last trick up his sleeve: Without the key in his hand, they'll never find the Princess.
Yoshi just snatches it off him, sending the Koopa King on a hasty exit. Luigi congratulates his new buddy, and questions him if he swallowed the key; thankfully not, and he spits it ever so conveniently into the keyhole, warping them back to Dome City with the Princess too. All thanks to Yoshi!
The story has lulled the dinosaur to sleep, so Luigi bids him good night, and is quietly called Mama Luigi before the episode ends. Aw. That's almost adorable.
This episode became part of a fad on YouTube of people editing episodes of cartoons to become five minute non sequiturs. The only one worth seeing, in my opinion, is the rap song.
Princess Peach has a whopping no lines of dialogue, and you never find out what happened to the cave people.