The play opens with young Bertram assuming the title of Count of Roussillon upon the death of his father. Helena is the orphaned daughter of a great doctor, and for years lived in the Roussillon household under the care of Bertram's mother, the Countess. Over the years, Helena has developed a secret love for Bertram, although she dares not tell him. The Countess, however, is well aware of Helena's feelings (and approves of them).
Against this backdrop, the King of France is taken deathly ill. Bertram has left to attend the King's court. Helena soon follows him to Paris, bearing a prescription of her father's that she feels might hold a cure for the ailing king. The cure earns her the gratitude of the King, who makes a bargain between them that she may have her pick of the bachelors at his court. Helena, of course, picks Bertram, who is quite dismayed by the prospect. To Bertram, Helena is beneath him and unworthy of his notice. Nevertheless, the King will keep his word—Bertram is ordered to marry her. Bertram assents to the marriage under protest, but then slips off to a war in Tuscany with his cowardly companion, Parolles.
Helena returns to Roussillon and the Countess, at first assuming that Bertram follow. It becomes apparent that he will not, and Bertram sends word that she may not call him husband until she gets from him a ring (which he always wears) and can bear him a child—not a simple task, especially given that Bertram is in Italy with no intention of ever consummating their marriage. Helena once again takes matters into her own hands and sets out to follow him. She arrives in Florence in the guise of a pilgrim and lodges with a widow whose daughter, Diana, is ironically the newest object of Bertram's affections. With Diana's help, Helena aims to trick Bertram, and thus is born one of the more infamous ploys in Shakespeare's repertoire: the bed trick.
Helena gets Diana to accept Bertram's advances. Bertram, however, must agree to give Diana his ring before they share a bed. At the crucial moment, Helena takes Diana's place in the dark. She also exchanges a ring that the King had given her for Bertram's, accomplishing both terms of Bertram's challenge. When a rumor is spread of Helena's death, Bertram decides that he is clear of any responsibility for a wife he never wanted, and he returns to France. However, the King easily recognizes the ring he bears as Helena's; when Bertram is caught in a series of lies, the King has him arrested on suspicion of murdering Helena. Adding to Bertram's misery, Diana and her widow mother arrive demanding justice. Helena finally appears—bearing Bertram's ring and carrying his child—and reveals the truth to all. With that, Bertram seems to repent of his wrongdoings and avows his dear love for Helena.