How do you listen to a four-hour opera? Tom Service considers the extraordinary impact of Wagner's opera Tristan und Isolde, a medieval romance that became in Wagner's hands a highly-charged erotic drama of unfulfilled longing. It scandalised and over-excited early audiences in the 1860s, and it still has a profound effect on listeners. How come? Tom explores the influence of the philosopher Schopenhauer on Wagner's thinking, and how the composer's own love-life may have influenced this piece. And musicologist Kenneth Hamilton takes Tom through the radical musical structures in this piece, which somehow manage to remain unresolved over long stretches of music. Did one special chord really change music forever?