It took me a minute to remember why, exactly, Prince Seon and Hwa-goon were skulking about in the middle of the night trying to steal something. That's because this whole subplot is nestled inside another subplot, the one about the merchant expedition, and even that quickly manages to turn into another subplot as Cheong-eun runs into Moo-ha (played by Bae Yoo-ram), an old ally with an exceptionally good memory. A pity that's about his only useful skill.
Meanwhile, Ga-eun decides to take revenge on the king through a really stupid plan that comes right out of a video game and of course works because the palace guards are not terribly competent. We already knew that, of course, considering that Dae-mok's private army can apparently waltz in there whenever they please, kill whoever they want, and face no ill consequences. Anyway, the other Seon reacts to this situation by agonizing over whether or not he should take his mask off.