Helena, the low-born ward of a French-Spanish countess, is in love with the countess's son Bertram, who is indifferent to her. Bertram goes to Paris to replace his late father as attendant to the ailing King of France. Helena, the daughter of a recently deceased doctor, follows Bertram, ostensibly to offer the King her services as a healer. The King is sceptical, and she guarantees the cure with her life: if he dies, she will be put to death, but if he lives, she may choose a husband from the court.
The King is cured and Helena chooses Bertram, who rejects her, owing to her poverty and low status. The King forces him to marry her, but after the ceremony Bertram immediately goes to war in Italy without so much as a goodbye kiss. He says that he will only marry her after she has borne his child and wears his family ring.
In Italy, Bertram is a successful warrior and also a successful seducer of local virgins. Helena follows him to Italy, befriends Diana, a virgin with whom Bertram is infatuated, and they arrange for Helena to take Diana's place in bed. Diana obtains Bertram's ring in exchange for one of Helena's. In this way Helena, without Bertram's knowledge, consummates their marriage and wears his ring.
Helena returns home to the countess, who is horrified at what her son has done, and claims Helena as her child in Bertram's place. Helena fakes her own death. Bertram, thinking he is free of her, comes home. He tries to marry a local lord's daughter, but Diana shows up and breaks up the engagement. Helena appears and explains the ring swap, announcing that she has fulfilled Bertram's challenge; Bertram, impressed by all she has done to win him, swears his love to her. Thus all ends well.
There is a subplot about Parolles – a disloyal associate of Bertram's: Some of the lords at the court attempt to get Bertram to know that his friend Parolles is a boasting coward—as Lafew and the Countess have also said. They convince Parolles to cross into enemy territory to fetch a drum that he left behind. While on his way, they pose as enemy soldiers, kidnap him, blindfold him, and, with Bertram observing, get him to betray his friends, and besmirch Bertram's character.