They did ended up doing karaoke. As usual , he was good and unsurprisingly, she was great. Together however, they were fantastic.

He's heard of people singing duets and falling in love through the power of song—he thought it was all bull before this. So when Eunji excitedly pulls him up for a song, he was completely relaxed and unconcerned.

As it is, it wasn't even a love song.

As it turns out, his predictions were so wrong. 

Because he had fallen for her so hard after the first verse, he knew that his world had just came crashing down.