Luigi is getting Yoshi ready for bed. Yoshi asks Luigi to tell him a story, so Luigi tells him about the Mario Bros. and Princess Peach's first adventure in Dinosaur Land, and how they met Yoshi, shown in flashback. It begins with the trio arriving in Dome City after the events of Super Mario Bros. 3, not knowing that Bowser has secretly followed them. Peach is suddenly being captured by some Chargin' Chucks while Mario and Luigi are chased around by Bowser's minions, getting separated. Eventually, a Sumo Brother cracks the ground and sends Luigi underground.
After slowering his descent using a P-Balloon and riding on a skull raft, Luigi looks for a Power-Up in some Prize Blocks, but ends up with a Yoshi Egg that hatches in his arms. Although Luigi doesn't think much of the crying newborn Yoshi (who mistakenly believes him to be his mother, hence the episode's title), he takes responsibility of him and keeps him safe from two Tyrannosaurus in the chasm. After avoiding all the aquatic enemies in Soda Lake and being brought to the Forest of Illusion, the two are surrounded by Wigglers, which Yoshi promptly chows down on, to Luigi's surprise. At this point, Mario shows up and, after almost getting eaten by Yoshi because he thought Yoshi was attacking Luigi ("Now I know how a meatball feels"), informs him that he just escaped from Bowser's Coney Island Disco Palace (or the Neon Castle for short), and Peach is still being held there. When Yoshi mean "Mama Luigi", Mario laughed at that while Luigi was a little embrassed.
When they arrive at Bowser's Castle, Yoshi gives the Marios some trouble by running off from them while they're trying to turn on the lights. Yoshi then hides from them in Bowser's Throne Room. When they pass by, Yoshi comes out from hiding and starts eating everything in there. Mario and Luigi are then chased back to the throne room by Bowser, who has sent some Mecha-Koopas after them. Bowser is angered at seeing his throne room empty and becomes shocked when Yoshi eats his Mecha-Koopas. Mario tells Bowser that unless he releases Peach, Yoshi will devour the whole castle. Bowser tries to escape with a key, but Yoshi snatches from him, causing him to run away empty-handed. Yoshi then spits the key (it tastes bad) into a nearby keyhole which, apparently, brings the Marios, Peach, and Yoshi back to Dome City. The flashback ends here, as Yoshi provides himself a laugh by calling Luigi his mama one more time.
