图穷匕首见

idiom

when the map is unrolled to the end, the dagger is revealed; (fig.) the true intention or hidden agenda is finally exposed

Allusion to the Warring States period assassin Jing Ke, who hid a dagger inside a map scroll to kill the King of Qin. When the map was fully unrolled, the dagger appeared. Modern usage often carries a negative connotation of concealed motives being unmasked.