The male lead and female lead had met in high school, meaning they were both of the same age. It is not necessarily a sexual relationship, but the male lead's age and appearance had been stunted in the tunnel, so he was 17-18 mentally and physically. Time passes a lot faster outside of the tunnel, in this case, about 14 hours in the tunnel would be (maybe 8) 13 years in real time. The female lead, who had stayed out of the tunnel, is now ~25 years in age, and kisses the male lead—who is still 17-18 mentally and physically—at the end of the movie.