This play is all about the "how," not the "what." The story's a bit thin, but there's not a plot-point in this play that isn't introduced, embellished, or concluded with a round of wordplay.
The King of Navarre and his lords Berowne, Longaville, and Dumain meet and swear off food, women, and sleep in order to study for three years. Berowne, the most independent of the four friends, isn't so keen on this whole idea. He reminds the King that the Princess of France is visiting on business. And what are they going to do for fun, anyway?
They'll be entertained by Armado, a Spanish soldier full of stories told in the bizarrely verbose style first introduced by a letter. He's tattling on the villager Costard for his entanglement with dairymaid Jaquenetta. The King sentences Costard to a week with only bread and water.
Armado is with his spicy page, Moth, sighing for love of Jaquenetta. She enters with the constable Dull and Costard. She's not so interested in Armado, so he takes his anger out on Costard and imprisons him. Giving up his warring ways, Armado sits down to write a love letter.
The Princess of France arrives on business, with her ladies Rosaline, Katharine, and Maria. Having heard something about the King's vow against women, she sends Lord Boyet as scout. She kills time by asking her ladies about the other lords of Navarre. It turns out that each lady has a crush on one of the lords. The noblemen emerge. Each connects with a woman and asks Boyet for her name. Boyet informs the Princess that, though the King won't grant her entrance to his court, he has a serious crushed on her.
Meanwhile, Armado is breaking the no-women-in-Navarre rule by sending out Costard with a love letter for Jaquenetta. Berowne has the same idea for his letter to Rosaline. Naturally, Costard mixes the notes up, and the noble ladies – busy hunting – end up with Armado's ridiculous ode to Jaquenetta's "heroical vassal."
Meanwhile, schoolmaster Holofernes and country preacher Nathaniel discuss the Princess's kill with Dull. Illiterate Jaquenetta enters with Berowne's letter, and Nathaniel reads it aloud. The two learned men critique the poetry.
Unsuspecting that his letter fell into the hands of a milkmaid, Berowne delivers a monologue wondering that he can fall in love. He's interrupted by the approach of the sighing King, and hides in a tree. He eavesdrops as the King reads aloud the sonnet he's written for the Princess. Then Longaville enters and the King hides and listens. Dumain enters and Longaville hides. One by one the men emerge, scolding each other for breaking their oaths. Berowne almost gets away without revealing his love, but Jaquenetta and Costard have excellent senses of timing. They enter just then with Berowne's letter, proof of treason. Berowne finally convinces the others that there's nothing wrong with wooing women and that it actually can be educational.
At the King's request, Armado organizes a pageant called the "Nine Worthies," with Costard, Moth, Holofernes, Nathaniel, and even Dull taking part. But first, the noblemen visit the women in the guise of Russians. Boyet has tipped the women off and they play a trick, confusing their identities by swapping favors (gifts the men gave them). Humiliated and turned out, the men return as themselves only to be scorned again. The nobles enjoy the pageant of the Nine Worthies – or rather, they enjoy heckling the incompetent actors – until they are interrupted by the arrival of the messenger Marcade from France. The King of France is dead, and the Princess and her ladies will have to return immediately.
The King of Navarre begs her to commit her love to him, but she says it's too soon to make such a decision. She asks him to spend a year in a monastery. If privation doesn't kill his love, she'll have him. The other ladies exact similar promises from their suitors.
The play ends with one last performance, the Song of Winter and Spring.