"What do you mean Yuna is in trouble?" Yerin asked, "I know, she's mountain climbing with the rest of the girls, but I believe they should be fine. Remember, Sowon and Eunha are practically experts in mountain climbing too." 

The girls, with the exception of Yerin and Yewon, were in Nepal, trying to reach the summit of the world's tallest mountain. Yewon, as much as she would like to, couldn't go because she had to stay to prepare the pitch to score a business deal with one the biggest players in the tech industry. Yerin was more than happy to volunteer to stay behind with her due to her acrophobia. 

Yuna, surprisingly, despite the fact that she's an acrophobic too, was the one who planned this expedition. She had it all planned out; she was going to film herself declaring her love for her dear best friend, at the summit, overcoming her fear of height. She believed that this was how she would definitely impress the love of her life, Yewon, when she finally confesses her ardent love to her. Everyone, except Yewon of course, are in on Yuna’s plan.

"And knowing you, you probably hid a tracker in Yuna's belongings again, didn't you?" Yerin added, to which Yewon nodded her head.

Yewon had always supported her best friend in all her exciting adventures, albeit it not being her interest. She had always enjoyed hearing the older go on and on about her exhilarat