Raise the Red Lantern

The film is set in 1920s China during the warlord era, years before the Chinese Civil War. Nineteen-year-old Songlian ( Sònglián, played by Gong Li), whose father has recently died and left the family bankrupt, marries into the wealthy Chen family, becoming the fourth wife or rather the third concubine or, as she is referred to, the Fourth Mistress ( Sì tàitai) of the household. Arriving at the palatial abode, she is at first treated like royalty, receiving sensuous foot massages and brightly-lit red lanterns, as well as a visit from her husband, Master Chen (Ma Jingwu), the master of the house, whose face is never clearly shown.Songlian soon discovers, however, that not all the concubines in the household receive the same luxurious treatment. In fact, the master decides on a daily basis the concubine he will spend the night with; whomever he chooses gets her lanterns lit, receives the foot massage, gets her choice of menu items at mealtime, and gets the most attention and respect from the servants. Pitted in constant competition against each other, the three concubines are continually vying for their husband's attention and affections.The First Mistress, Yuru (Jin Shuyuan), appears to be nearly as old as the master himself. Having borne a son decades earlier, she seems resigned to live out her life as forgotten, always passed over in favor of the younger concubines. The Second Mistress, Zhuoyun ( Zhuóyún, Cao Cuifen), befriends Songlian, complimenting her youth and beauty, and giving her expensive silk as a gift; she also warns her about the Third Mistress, Meishan ( Méishn, He Caifei), a former opera singer who is spoiled and who becomes unable to cope with no longer being the youngest and most favored of the master's playthings. As time passes, though, Songlian learns that it is really Zhuoyun, the Second Mistress, who is not to be trusted; she is subsequently described as having the face of the Buddha, yet possessing the heart of a scorpion.Songlian feigns pregnancy, attempting to garner the majority of the master's time and, at the same time, attempting to become actually pregnant. Zhuoyun, however, is in league with Songlian's personal maid, Yan'er ( Yàn'ér, played by Kong Lin) who finds and reveals a pair of bloodied undergarments, suggesting that Songlian had recently had her period, and discovers the pregnancy is a fraud.Zhuoyun summons the family physician, feigning concern for Songlian's "pregnancy". Doctor Gao ( Go-yshng, Cui Zhigang), who is secretly having an illicit affair with Third Mistress Meishan, examines Songlian and determines the pregnancy to be a sham. Infuriated, the master orders Songlian's lanterns covered with thick black canvas bags indefinitely. Blaming the sequence of events on Yan'er, Songlian reveals to the house that Yan'er's room is filled with lit red lanterns, showing that Yan'er dreams of becoming a Mistress instead of a lowly servant; it is suggested earlier that Yan'er is in love with the Master and has even slept with him in the Fourth Mistress' bed.Yan'er is punished by having the lanterns burned while she kneels in the snow, watching as they smolder. In an act of defiance, Yan'er refuses to humble herself or apologize and thus remains kneeling in the snow throughout the night until she collapses. Yan'er falls sick and ultimately dies after being taken to the hospital. One of the servants tells Songlian that her former maid died with her mistress's name on her lips. Songlian, who had briefly attended university before the passing of her father and being forced into marriage, comes to the conclusion that she is happier in solitude; she eventually sees the competition between the concubines as a useless endeavor, as each woman is merely a "robe" that the master may wear and discard at his discretion.As Songlian retreats further into her solitude, she begins speaking of suicide; she reasons that dying is a better fate than being a concubine in the Chen household. On her twentieth birthday, severely intoxicated and despondent over her bitter fate, Songlian inadvertently blurts out the details of the love affair between Meishan and Doctor Gao to Zhuoyun; who later catches the adulterous couple together. Following the old customs and traditions, Meishan is dragged to a lone room on the roof of the estate and hanged to death by the master's servants.Songlian, already in agony due to the fruitlessness of her life, witnesses the entire episode and is emotionally traumatized. The following summer, after the master's marriage to yet another concubine, Songlian is shown wandering the compound in her old schoolgirl clothes, having gone completely insane.

Question: With whom is the third mistress having an affair?
Answer: Doctor Gao