Set in a Spanish prison, near Seville, Fidelio , otherwise Leonore, oder Der Triumph der ehelichen Liebe (Leonora, or The Triumph of Married Love), centres on the loyalty and love of Leonora, who disguises herself as a boy, Fidelio, and takes service under the gaoler Rocco, discovering in the deepest dungeon her husband, Florestan. Don Pizarro, who holds Florestan prisoner for personal reasons, orders his secret murder, which Leonora is able to prevent as the arrival of the minister Don Fernando is announced, an event that will put all to rights again. Additional complications for Leonora include the unwanted affection for her of Marzelline, who is unaware of her true identity, and the consequent rivalry of Jaquino. Rocco, a good man who obeys orders, however unjust, may be seen as a representative figure, while the prisoners in his charge have their moment in the famous prisoners' chorus that allows them to emerge for a moment into the fresher air.
The four overtures to the opera, the three earlier Leonora overtures now generally replaced by the 1814 Fidelio overture, are heard often enough in the concert-hall, in particular the third of the Leonora overtures, which anticipates the climax of the opera by the use of an off-stage trumpet heralding the arrival of the deus ex machina , Don Fernando. The prisoners' chorus, O welche Lust (Oh what pleasure), in the second scene of the later two- act version of the opera, is remarkably effective, while other vocal excerpts include the quartet for Rocco and his household, Mir ist so wunderbar (It is so wonderful for me), Leonora's horrified recitative and aria Abscheulicher! Wo eilst du hin? (Detestable man! Where are you hurrying to?) and Florestan's moving Gott! welch'Dunkel hier! (God! How dark it is here), as he lies shackled in his underground cell.
The work is a Singspiel, with spoken German dialogue. Its initial failure, owed in part to the French occupation of Vienna at the time, led to an immediate revision, with the 1814 version finding greater continuing favour. While the musical influence of Mozart may be perceived, and possibly of other treatments of the subject by the composers Gaveaux and by Paer, the plot itself owes much to French revolutionary opera, the work of Cherubini and Méhul.