RewriteEngine On

RewriteCond         %{REQUEST_FILENAME}  -f [OR]
RewriteCond         %{REQUEST_FILENAME}  -d
RewriteRule  ^(.+)  -  [L]

RewriteCond         /alt_dir/%{REQUEST_FILENAME}  -f [OR]
RewriteCond         /alt_dir/%{REQUEST_FILENAME}  -d
RewriteRule  ^(.+)  /alt_dir/$1  [L]

RewriteRule .* /alt_dir/fail_script [QSA,L]