This second of the four overtures Beethoven wrote for the opera that would eventually become Fidelio was actually the one that introduced the work's first three performances in 1805, but it has never achieved the popularity of its successors. Even so, it's played in concert a bit more frequently than its predecessor.
It's a hefty work, of about the same duration and structure of the well-known Leonore Overture No. 3, but some of the themes are substantially different. A slow, foreboding introduction that clearly influenced the later overtures of Weber drifts forward in darkness without establishing a particularly strong melody, until the appearance of scalar figures in the woodwinds (familiar from the third overture). While the initial slow material continues, these figures flip bar by bar from the top of the orchestra (where they ascend) to the bottom (where they descend). Heavy chords bring an end to this, after which the woodwinds introduce a cautiously more optimistic theme, with the strings dwelling on a rhythmic figure related to the scalar motif. Gradually the orchestra builds up to the galloping main theme of the allegro section (melodically identical to that in the third overture, although its development is different). This and a broader, noble subsidiary theme dominate the second half of the overture, although at the climax the orchestra pulls up short for the offstage trumpet calls that, in the opera, announce the arrival of the king's representative, who brings justice to the imprisoned hero. After this episode, cascading string figures rev up the orchestra for the fast, full-bodied concluding pages, although the final, widely spaced chords make a far less thrilling effect than the linked and more concise chord sequence ending the third overture.