By the end of 1854, Wagner had sketched out all three acts of an opera on the Tristan theme, based on Gottfried von Strassburg's telling of the story. While the earliest extant sketches date from December 1856, it was not until August 1857 that Wagner began devoting his attention entirely to the opera, putting aside the composition of "Siegfried" to do so. On 20 August he began the prose sketch for the opera, and the libretto (or "poem", as Wagner preferred to call it) was completed by September 18. Wagner, at this time, had moved into a cottage built in the grounds of Wesendonck's villa, where, during his work on "Tristan und Isolde", he became passionately involved with Mathilde Wesendonck. Whether or not this relationship was platonic remains uncertain. One evening in September of that year, Wagner read the finished poem of "Tristan" to an audience including his wife, Minna, his current muse, Mathilde, and his future mistress (and later wife), Cosima von Bülow.