Their highest-selling UK single and their only US number one, some may consider this to be a tacky wedding song, but for other's it's a deeply moving work of art. From its origin in a chalet on an island off of Stockholm through to its first performance in front of the King of Sweden almost a year later, this episode charts its writing and recording process and you'll hear deleted sections of the song and, for the very first time, the acapella of the girls' incredible singing.