Yoo-mi heads back to the beachhead where this all began, and Ji-wook follows her, as the penultimate episode mostly focuses on nostalgia. And it really is nostalgia- I remember the first episode, and I also remember that it had a radically different tone than what the flashbacks here are pretending. Yoo-mi and Ji-wook were perpetually grumpy, and only bond at all because they were both extremely drunk.
But that's what makes their concerted effort to replace that goofy tone with a romantic one here all the sweeter. Yoo-mi and Ji-wook have been such a generally positive influence on each other that all of their past interactions are colored in the optimistic light of what their relationship is right now. That they are technically broken up at the moment is besides the point. Yoo-mi was drawn back to that beach in the first place precisely because her mood was so badly complicated by her mother's nefarious deeds.