So I watched again, and it is better second time through, easier to follow what's going on.
Madam Ching wanted the Keystone to use as treasure to buy her kidnapped crew back. She knew the statue of the Sea Devil had a clue to its whereabouts, so she came there and accidentally set the Sea Devil free. That same Sea Devil had been after the Keystone centuries before to use in its own plans, knew Ji-Hun had it, but Ji-Hun sent it away with his first mate. After putting Ji-Hun in suspended animation, the Sea Devil went after the first mate, but was defeated by an army and trapped in stone by the power of the Keystone. Now that he's released, he summons his Sea Devil crew who have been on board Ji-Hun's old ship working on his plan to mess up the Earth while they waited for him, and they go after the Keystone again. The Sea Devil gets it, but then it's lost when the Doctor implodes all the Sea Devil tech, so Ji-Hun gives Madam Ching some of his old treasure from his own ship to use to buy back her crew instead. Then Ji-Hun sacrifices himself to stop the Sea Devil because he should have died centuries ago anyway, and they all live lesbianly ever after.
There, that's the plot. I just think the writer or the director or both didn't do a very good job of making it clear what the actual plot was.