A social satire set in England and Scotland during the early 20th century, What Every Woman Knows centers around plain, spinsterish Maggie Wylie and John Shand, an ambitious young student, who promises to marry Maggie after five years if she agrees and if her family pays for his education. Years later, following his successful bid for a seat in Parliament, Shand keeps his word.

But trouble lies ahead. Attractive woman are drawn to the Scottish politician - in particular, the lovely Lady Sybil Tenterden. Moreover, Shand's speeches in Parliament, which had won him great popularity for their flashes of humor, begin to suffer when his wife no longer helps write them. Soon, both Shand's career and marriage are in jeopardy.