After slaying a tiger at Jingyang Ridge, the hero Wu Song discovered that his brother Wu Dalang had been poisoned to death by his adulterous wife Pan Jinlian and her lover Ximen Qing. In a fury, Wu Song killed both of them to avenge his brother. Carrying their heads, he turned himself in. The magistrate of Yanggu County sent him to Dongping Prefecture for sentencing. The prefect ordered the matchmaker Wang Po to be executed by slicing, then had Wu Song fitted with an iron cangue, his face tattooed with golden marks, and exiled to the prison camp in Mengzhou.
As Wu Song and the two escorts trudged toward Mengzhou under the scorching sun, they stopped for a drink at a tavern beneath a great tree at Crossroads Slope. The innkeeper, Sun Erniang, noticing Wu Song's heavy bundle, plotted to drug them with knockout powder. Suspicious of her lingering gaze on his pack, Wu Song feigned ignorance and remarked, "I've heard it said on the road: 'At Crossroads Slope beneath the tree, no traveler dares to stop for tea—the fat ones are minced for buns, the lean ones tossed into the stream.'" Sun Erniang naturally denied it, insisting her buns were filled with beef. Wu Song pressed further, claiming he'd found a pubic hair in the filling, then taunted, "Your husband's away—aren't you lonely?" Smirking coldly, she thought, "This exiled convict is asking for death, daring to mock me! Truly, 'a moth flying into the flame only burns itself.'"
Sun Erniang instructed her waiter to lace the wine with knockout drops, pouring three steaming bowls for Wu Song and his two escorts. The escorts, oblivious, gulped theirs down. Suspecting the wine, Wu Song feigned reaching for meat to distract Sun Erniang, then secretly emptied his bowl into a corner. As the two escorts collapsed, Wu Song pretended to be drugged too. Two waiters tried to lift him but couldn't budge him. Scolding them as useless, Sun Erniang herself lunged to hoist Wu Song, intending to butcher him for human-meat buns. In a flash, Wu Song seized her, pinned her beneath him, and she squealed like a stuck pig, begging for mercy.
Fortunately, Sun Erniang's husband, Zhang Qing, returned home just in time. After asking Wu Song to explain the situation, Wu Song finally released Sun Erniang. Embarrassed, Sun Erniang then sheepishly explained the whole story behind the incident.
"The moth flies into the flame" — also written as "the moth dashes into the fire" or "the moth plunges into the blaze" — describes seeking one's own doom and courting destruction.
Source: *Water Margin*
Meaning of the Idiom: Later, the Chinese idiom "飞蛾扑火" came to describe seeking one's own doom and courting destruction.