Seen from the perspective of a window washer who is one of the main characters in “Lost in Beijing,” the Chinese capital is an endless vista of glassy high-rises. Closer to the ground, in the lyrical documentary montages that frame Li Yu’s modern urban melodrama, the city is a hive of human work and pleasure seeking. There are tango clubs and dive bars, car dealerships and open-air markets, nouveau riches in their Mercedes-Benzes and homeless people stretched out on benches.

There are, at last count, something like 17 million stories in this naked city, and Ms. Li, who wrote the screenplay for “Lost in Beijing” with Fang Li, relates a tale that is at once representative of the social and economic tensions afflicting 21st-century China and ripely, improbably melodramatic. The director and her cast work in a rough, naturalistic style, but the narrative offers both the pleasures and the limitations of old-fashioned class-conscious pulp. In spite of its raw, explicit moments, the film is at heart a sturdy morality tale about innocence and corruption, wealth and want, sex and power.

An-kun (Tong Da Wei), the window washer, is married to Ping-guo (Fan Bingbing), who works at the Gold Basin Massage Palace rubbing the feet of tired businessmen and deflecting their roving hands. Her boss, Mr. Lin (Tony Leung Ka Fai), is a rough character as yet unpolished by money, an unapologetic gambler and womanizer. One afternoon he finds Ping-guo passed out in his office, and his attempt to take advantage of her turns from a clumsy pass into a rape. An-kun, who happens to be washing the window of Mr. Lin’s office and witnesses the assault — this is what I mean by improbable — tries first to attack the older, more powerful man and then to blackmail him.