Giant Chess Extravaganza: A shocker!

Hello again!  During finals week at the UofC, the Uncommon Fund sponsored a student initiative to bring giant chess pieces to the University.  I was lucky enough to get to play on it for a while.  It is quite a different experience from regular otb chess!  I found it surprising and edifying how a slight change in visual perspective impacts calculation and chess sight.  I played four very casual games outside in a courtyard on campus against some amateur players and went 4-0, though I think I still would have enjoyed it going 0-4.  Anyway, the instructional point of this post is to comment on how complacency in a winning position is dangerous, especially when there is tactical energy in the position.  Proceed with caution and see if you can find (before examining the notes and variations), white's shocking response after 21. Qa7?? :

 

 

   Don't stop calculating until the king is netted.  Thanks for viewing, and best of luck in your chess!