Egisto survives as the most representative and, in modest terms, most-often staged of Cavalli's 40-plus operas. In the style of the time, Cavalli classified L'Egisto as a "favola drammatica musicale," or musical dramatic fable. The libretto by Giovanni Faustini was drawn from Roman mythology, safely avoiding overt references to current events or personalities. Musically, Cavalli kept his lines clear. The work follows the standard format for Venetian opera of this period, including a prologue in which allegorical characters and gods prepare the audience for a drama played out in three acts. Here these are complemented by scenes at the end of the first two acts in which such allegorical characters return to comment on the central action. Although most scenes involve at least two characters, each character sings in turn rather than overlapping with others; the most significant exceptions are a sprightly duet between Venus and Amor at the end of the first act and music for Egisto and Clori in the opera's final pages. Singing alternates between extended recitative and a leisurely, florid, and sometimes doleful aria. Instrumentation was a matter of what was available, the assumption being there would at least be a modest body of string players and some sort of continuo group embellishing the spare bass line. Today's performers must decide whether to try to reproduce the forces available at a specific theater in the 1640s or to assemble some idealized orchestra that's both historically valid and rich enough to suit modern tastes.

After an instrumental sinfonia, in the prologue, the character La notte (Night) temporarily cedes rule of the earth to L'aurora (Dawn). The main story commences with Act One, in which Cavalli's sweet, gentle music evokes an Arcadian setting. Clori and Lidio, a young nobleman, declare their love for each other while Climene, formerly engaged to Lidio, wakes Egisto, formerly engaged to Clori, from a nightmare. Under the hostile watch of Venus, goddess of love, Egisto and Clori had been kidnapped and separated; the same fate has befallen Lidio and Climene, and now all four oddly find themselves together on the same island. The complications which follow include mischief made by Climene's brother Hipparco and the god Amor. Naturally, everything works out in the end, and under Amor's influence, the lovers return to their pre-opera pairings.

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