They run into a Japanese ship from which they seize a treasure map purporting to show the location of the lost treasure of the Kingdom of Goryeon and decide to go after it themselves.
This is big silly pirate nonsense. The two attractive leads bicker and flick their hair at each other. There's a comedy coward. Terrible CGI penguins. Cross and double cross. Some bad guy who "has a history" with the hero. Occasional martial arts set pieces. Slapstick. Romance. A princess. An emo archer. Horrible food. Flitting from scene to scene with no narrative to bridge the gap. A volcanic eruption. Improbable aqualungs made from pigs bladders. Lots of CGI jellyfish. Handbrake turn in a pirate ship. Stirring music. A stampede of cows. The old guy who cries doom at every suggestion. A tiger they didn't have the budget for. Swordfight on a mountaintop.
On and on in a lengthy cavalcade of whatever they could chuck in and get away with.
It's quite good fun, I found myself laughing along with it. Nowhere near as polished family entertainment as something like Jungle Cruise but boy do they pack a lot in and there are a LOT of characters who all get to do stuff and a LOT of scenes. Worth a look 7/10 mostly because it does that Korean cinema thing of being not quite like the rest of the US commissioned/financed stuff lurking on Netflix.