If Hidden Blade targets an opponent's unit and they chain Retread to return it to hand, does the opponent still draw 2 cards?

Ruling: No, the opponent does not draw 2 cards. When the targeted unit is returned to hand before Hidden Blade resolves, "its controller" becomes unknown so nothing happens.

Nuances:

• The effect fails to resolve because the game can no longer determine who the controller of the unit is once it leaves the battlefield