The libretto is based on a poem by Pushkin. It's all about the love between a Prince and a Miller's daughter, which in the class system of the time could not be. The Prince marries another more suited to his station, regrets it and in the end drowns in his attempt to find his beloved who has turned to a mermaid.
The Brilliant set does not include a full libretto, which is a shame, but the music is so good that one overlooks it after a spin or two. Dargomyzhsky is a cross between a kind of Russian Rossini and a precursor to Mussorgsky in the lyrical and melodic memorability of the music and its sometimes folksy quality.
The performances excellent, the sound very good and the music a real find for those like me who had not heard the opera previously. If you follow the history of the Russian classical style this one is hands-down a must.