It's been several weeks since Toothless was transformed into that new very human-like and womanly version of him-... of *herself*. Hiccup and his friends have been looking everywhere for clues about how to turn the nightfury back to their original form, with no conclusive result.
But one day, during the month when merchants from the south come to Berk's harbor, Hiccup met one who claimed to know a thing or two about transformation magics. He was from a far, far away land, his accent was so thick it made his speech hard to understand for the young viking. But Hiccup was so eager to finally find a way to get his dragon back to normal. He grabbed the potion that was presented to him, and interpreted the foreigner's gesture as if he asked him to drink it, bottoms up! Less than seconds later, before everyone's dumbfounded eyes at the market, Hiccup's body changed. His legs grew longer and hips wider, popping his belt and making his pants fall. His shoulders narrower as he took a nice hourglass figure, and a shapely pair of breasts ripped out his shirt. The merchant on his end, with a satisfied nod, demanded his payment. It was a bit too late for the poor viking to realize the misunderstanding.
In the end, the merchant had nothing in store that could help Toothless case. While no solution was found, the problem was actually doubled! Astrid will have work teaching her new girlfriend about everything she'll have to deal with. For now at least, unless... there was no way back? There had to be one! Hiccup was still to be Berk's chief! And the chief *had* to get an heir... one way or another.