When the Map is Unrolled, the Dagger is Revealed
In the end of the Warring States Period（475－221 BC）, there was a strong country named Qin. It wanted to annex other countries to unite China. In 228 BC, the Qin army captured a country and prepared to attack another country named Yan. So the prince Dan of Yan sent a brave man called Jing Ke to assassinate the King of Qin.
In order to gain the confidence of the King of Qin, he carried a map of the most fertile part of Yan territory to Qin and hid a sharp dagger in it. He came to Qin and entered the palace. When the king ordered the map be presented, he took out the map and presented it to the king with great respect. Slowly, he unrolled the map. When it was completely unrolled, the hidden dagger appeared. Jing Ke grabbed it and stabbed at the king. But the king dodged the dagger and hid himself behind a big pillar. Jing Ke had never got the chance to kill the king as he was soon killed by the palace guards. The plan has failed. Later, Qin had wiped off Yan.
The idiom describes the real intention is revealed in the end.