So there's this nefarious fellow mining some gem dust from the bottom of the ocean for a plan never really described. He's nabbing folks from ships and boats by way of a multi-armed robot that folks often mistake for a pack of sea serpents. He nabs AstroBoy, and AB meets some of the folks who've been there for years, two of whom are Princess Papaya Petal and her father. Although King Petal knows that AB's a robot, Miss Petal never figures it out. So when AB's overcome by rust and rime, she's positive he's coming down with some horrible illness.
AB comes up with a plan to sneak folks out of the undersea mining base by storing them in empty bottles, but beating up the base's arms after getting everyone out only makes him more overcome by rust and rime. They all end up back at where-ever Princess and King Petal are from, with Princess Petal concerned about AB's health. The king and AB realize that all they have to do is mail AB back to Professor Elefun, so they contrive some hospital story for Princess Petal (AB really is keen on the idea of someone mistaking him for human, since that's his raison d'etre in a lot of ways) and ship him off.
Cut to a year later after oodles of hot-oil baths. King and Princess Petal head to the institute to visit, but Papaya Petal apparently got it in her head that AB must've died (since he never wrote back), and King Petal thought it best, so this way she'll never realize AB is "just" a robot. So AB hides behind a hill, looking sullen, as the King and the Professor show Princess Papaya Petal around, talking about what AB "used to enjoy".
Roll credits. Apparently the moral of the story is that it's better to conclude an optimistic lie with a romantic lie than to just tell a disillusioning truth.