"Goodbye Summer" is a heartfelt collaboration between f(x)'s Amber, Luna, and Krystal, with the help of D.O. from EXO. The song talks about the bittersweet feeling of summer coming to an end and reliving precious memories before bidding farewell to the season. The lyrics, combined with the trio's angelic voices and D.O.'s smooth rap, make for a perfect soundtrack to summer's end. #BehindtheMusic