媽閣是座城是一部劇情片，梅曉鷗（白百何飾）在媽閣這座賭城中輾轉謀生。爲生計，她做疊碼仔，出入賭場，過著放債與討債的生活，看遍形形色色的賭徒；爲愛，她孤注一擲，賭上全副身心。 在這裏她痛失過婚姻，與丈夫盧晉桐（耿樂飾）決裂，又接連遇見了風度翩翩的段凱文（吳剛飾）、與她的世界格格不入的藝術家史奇瀾（黃覺飾）。 梅曉鷗在一場場愛情賭局中，推上過青春、孩子、婚姻，甚至身家性命作爲籌碼，最終她能否贏得一段完完全全的真感情？而賭桌對面的三個男人，又將面臨怎樣的輸贏？
Macau's return to Chinese sovereignty in 1999 ushered in that city's "golden age." By 2012, the value of Macau's gaming industry was nine times that of Las Vegas, and Macao had become the world's foremost gambling destination. During this period, a unique profession arose: "casino brokers," who provided Macao's gamblers with high-interest lines of credit, under-the-table bets, and other services. It was a high-stakes, high-reward business that made for more thrilling play, but amplified the risks for players, brokers and casinos alike. This heightened risk made gambling more exciting and more dangerous for the participants; it also impacted their interpersonal relationships and outlook on life. It was as if everyone involved were walking on a tightrope. By 2014, however, China's ongoing anti-corruption campaign had dealt a serious blow to the gaming industry in Macao, ending the golden age. This is the background against which our story takes place.