Amir apparently began plotting against Hassan on the night of his birthday party. Before he went to bed, Amir asked Baba if
... he'd seen my new watch anywhere.
Amir knew where it was: He had tossed it on his pile of gifts shortly after receiving it, not even bothering to put it on. The next morning, Amir waited until Ali had completed all of the cleaning. When Amir looked out of his bedroom window and saw Ali and Hassan heading to the bazaar to shop for groceries with their wheelbarrows, Amir knew it was time to make his move. Amir made sure that Baba was busy in his office, talking on the phone, and then he went to Ali's quarters
... by the loquat tree. I lifted Hassan's mattress and planted my new watch and a handful of Afghani bills under it.
Amir waited a half-hour before informing Baba that his gifts were missing. When Ali and Hassan returned, they were asked by Baba to come to his office, where a terrible surprise awaited them.