Comments from wikipedia Opera game page
1. e4 e5 2. Nf3 d6 3. d4 Bg4(?)
Though censured today, this was standard theory at the time. Now 3...exd4 or 3...Nf6 are usual. 3...f5 is a more aggressive alternative.
4. dxe5! Bxf3
If ... dxe5, then 5. Qxd8+ Kxd8 6. Nxe5 and White wins a pawn and Black has lost the ability to castle. Black, however, did have the option of 4...Nd7 5.exd6 Bxd6, when he's down a pawn, but has some compensation in the form of better development.
5. Qxf3 dxe5 6. Bc4 Nf6
This seemingly sound developing move runs into a surprising refutation. After White's next move, both f7 and b7 will be under attack.
7. Qb3! Qe7
Black's only good move. Almost all of Black's moves lead to checkmate, for example 7. ... Nc6 8. Bxf7 Ke7 9. Qe6#. 7. ... Qd7 loses the rook to 8. Qxb7 followed by 9. Qxa8. Notice that Qe7 saves the rook with this combination: 8. Qxb7 Qb4+ forcing the queen exchange saving the rook. Black is forced to move the queen to e7 which blocks the f8 bishop and more importantly impedes kingside castling.
8. Nc3!
White prefers fast development to material. He declines to win a pawn with 8. Qxb7 Qb4+ (the only way to avoid loss of the rook) 9. Qxb4, or to win two with 8. Bxf7+ Kd8 (or 8. ... Qxf7 9. Qxb7 and now Black cannot avoid loss of the rook) 9. Qxb7, preferring to mass his forces for a quick checkmate and get back to the opera.
8. ...c6 9. Bg5 b5? 10. Nxb5!
Morphy chooses not to retreat the bishop, which would allow Black to gain time for development. Black's move 9. ... b5 loses but it is difficult to find anything better; for example 9 ... Na6 10.Bxf6 gxf6 11.Bxa6 bxa6 12.Qa4 Qb7 and Black's position is in shambles.
10... cxb5
Black could have played 10...Qb4+, which would have forced Morphy to exchange queens, although White would retain a clearly won game
11. Bxb5+ Nbd7 12. 0-0-0
The combination of the bishop's pin on the knight and the open file for the rook will lead to Black's defeat.
12... Rd8
13. Rxd7! Rxd7 14. Rd1
Compare the activity of the White pieces with the idleness of the Black pieces.
14... Qe6
Qe6 is an attempt to unpin the knight but the game is lost here.
15. Bxd7+ Nxd7
If ... Qxd7, then 16. Qb8+ Ke7 17. Qxe5+ Kd8 18. Bxf6+ gxf6 19. Qxf6+ Kc8 20. Rxd7 Kxd7 21. Qxh8 and White is clearly winning.
16. Qb8+!
Morphy finishes with a stylish queen sacrifice.
16... Nxb8 17. Rd8#