The Princess's Substitute - S1.E6

The Princess of Marlo, Claris, who bears a striking resemblance to Sherlock, doesn't want to get married to a prince in order to support their country. Wanting to be a normal girl, she runs off and finds Sherlock and offers to trade lives with her for a day. While Claris hangs out with the rest of Milky Holmes, Sherlock spends the day with her prince-to-be, Pero, who is a masochist with a catgirl maid fetish. As Claris contemplates staying in this new life, her identity is discovered by Henriette, who tells her to stop running away from her troubles. Getting the help of Milky Holmes, Claris gets to the church before Sherlock gets wed to Pero, revealing she will reach her dreams with her own hands. Pero reveals he had been trying to get her to put off the marriage, as he was in love with her grandmother, who he marries, connecting the two countries after all.