Toward the end of the Warring States Period (475－221 BC), there was a powerful state named Qin. It wanted to annex other states to unite China. In 228 BC, the Qin army conquered one of the states and prepared to attack another named Yan. So the Yan prince, Dan, sent a brave man called Jing Ke to assassinate the king of Qin.
In order to gain the confidence of the King of Qin, Jing Ke brought a map of the most fertile part of Yan territory for the king and hid a sharp dagger in it.
Jing Ke was allowed to present the map to the king in the palace. With great respect, he unrolled the map slowly. 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 was then killed by the palace guards. His plan failed. Later, Qin wiped off Yan.
People use the idiom to indicate that the real intention is revealed in the end.