Mortimer responds before you have a chance to, by flinging himself at the stranger.
“Oh, thank goodness you’re here, Father!” he declares. “This brute forced me to help him in his depraved schemes! He even made me attack the smith’s dear old mother, and kidnap this innocent grebling!”
“Father” seems thrown off by the declaration, but his eyes turn immediately to the unconscious grebling as Mortimer mentions them. You only note this in passing because you’re already running to the back room. Maybe if you press on the door hard enough, you can dislodge the box…
“Get ahold of yourself, boy!” you hear the man shouting as you duck inside. “Stanley, get that ledger! And the shield, while you’re at it. I don’t care what happens to the thief. Christine, go outside and try to cut him off in the back alley.”
Good thing he doesn’t seem to care how loud he’s barking orders. Now you know you’re going to be in trouble even if you do manage to get outside.
Which suggests your best bet is to figure out how to deal with Stanley. Is there anything back here that can help you?