He leaves to get tools to break open the dressing room door, giving Chérubin enough time to escape through the window and Suzanne time to take his place in the dressing room; when the Count opens the door, it appears that Suzanne was inside there all along.
In a comical style, Mozart leads this motif into a loud rumpus which involves the whole ensemble.
According to the reminiscences of Michael Kelly, the Irish who played Basilio in the premiere, the effects were so wonderful that in the first full rehearsal, the entire company spontaneously broke into cheers for the composer.
To secure the loan he agreed to marry Marcellina if he couldn't pay his debt.
Susanna leaves as the Countess, dressed as Susanna, comes back and waits for the Count while everyone else is hiding.