The countess Diana is enraged when she hears that a man was seen leaving the upper chambers of the palace. He threw his cap at the candle, snuffing out the only light so that he could not be identified. Diana sends for her ladies-in-waiting and questions them to learn who had been visited by a lover during the night. Dorotea and Anarda plead innocent but whisper to Diana that Marcella has a lover in the palace. He is Teodoro, secretary to the countess Diana herself. Marcella confesses her love but protests that it is a pure love. Teodoro wants to marry her. Diana gives her consent to the marriage but cautions Marcella to stay away from Teodoro until the wedding day; otherwise passion might consume honor. After her ladies leave her alone, Diana realizes that she, too, loves Teodoro, but since he is not highborn she cannot proclaim her love.

Teodoro, who had indeed been the man involved in the midnight escapade, fears that he will be found out and banished or executed, but he cannot get Marcella out of his heart. Tristan, his lackey, begs him to forget Marcella and never see her again lest Diana punish them; it is Tristan who threw the cap and snuffed out the candle so that his master would not be recognized while escaping. Soon afterward, Diana tricks Tristan into revealing his part in the affair; she also sends for Teodoro and subtly hints at her love for him in a letter she feigns is intended for someone else.

Marcella goes to Teodoro and tells him that Diana has blessed their betrothal. Confused, Teodoro takes Marcella in his arms just as Diana appears. When he thanks her for giving Marcella to him, their capricious mistress orders Marcella locked in her room to await her decision concerning the wedding. Then Diana again hints to Teodoro that she loves him, whereupon he renounces Marcella. He regrets rejecting Marcella, but he cannot put aside the lure of wealth and power that will be his if Diana takes him for a husband. After Marcella is released from the locked room, Teodoro, meeting her, spurns her love and disgraces her. Marcella swears revenge on him and on Anarda, who has, as she learns, betrayed her and Teodoro to Diana because Anarda thinks Marcella has been encouraging Fabio, a gentleman with whom Anarda is in love. Marcella, meeting Fabio, offers him her love and greatly confuses that poor man by her words and actions.

When two noblemen, the Marquis Riccardo and Count Federigo, both beg for Diana’s hand, she sends Teodoro to tell Riccardo that she chooses him for her husband. Deserted by the lovely countess before she is really his, Teodoro turns back to Marcella and tells her that he loves only her. At first she spurns him and declares she will marry Fabio, but at last love wins over jealousy. Falling into Teodoro’s arms, she makes him forswear Diana forever. The lovers call their mistress a devil, an ass, and a bore, not knowing that Diana and Anarda are hidden nearby and listening to their conversation. When they suddenly appear, they frighten the lovers almost to death. Diana dictates a letter to Teodoro, in which she states that if a noble lady loves a man, he dare not love another. When she cautions him to interpret its meaning correctly, Teodoro again renounces Marcella and tells her to marry Fabio so as to please Diana.

Riccardo, appearing in answer to the summons from Diana, is told that Teodoro misunderstood her words and that she did not intend to marry him. Teodoro, believing then that his mistress truly loves him, declares his love for her. Instead of listening to his pleas, Diana berates him for daring to speak of love when he is lowborn and she a lady. As she does not intend to have him herself, he thereupon asks her for Marcella. Like the gardener’s dog who will allow no other dog to eat what he himself does not want, she refuses to let Teodoro have Marcella. Instead, she strikes at Teodoro with her knife. He half-believes that she wounds him because she loves him, and, when she returns and wipes the blood from his wound, he is sure that it is love that makes her cruel to him.

Count Federigo and the Marquis Riccardo, hearing that Diana has wounded Teodoro, are convinced that he had threatened her honor, and they decide to have him killed. For their assassin they hire Teodoro’s faithful lackey, Tristan, who takes their gold and then informs Teodoro of their plot. Tristan has other plans for helping his master. He learns of one Count Lodovico, who had lost a son named Teodoro twenty years before. The boy had been captured by the Moors and was never heard of again. Tristan plans to convince the old count that Teodoro is his long-lost son. Then Teodoro will have a family of birth and wealth and will be good enough to wed Diana. Teodoro, too honorable for such knavery, goes to Diana and tells her that he is going to Spain, to avoid both the death planned for him by her suitors and the torture he endures while in her presence. Diana, not knowing her own mind, alternately tells him to leave and to stay. When Marcella goes to Diana and asks for permission to accompany Teodoro to Spain, Diana tells the girl that she must marry Fabio.

Meanwhile, Tristan carries through his plot to make Count Lodovico think that Teodoro is his lost son, and the old man is delighted at the prospect of having his child returned to him. Before the old count sees Teodoro, Diana, knowing that her true love is to leave her, tells him at last that she loves him. Still she refuses to marry him because of his humble birth. When Count Lodovico appears with the announcement that Teodoro is his son, Diana opens her arms to Teodoro and says that they will be married that very night. Marcella, finally realizing that she can never have Teodoro, agrees to marry Fabio.

Teodoro, in one last attempt to save his honor, confesses to Diana that Tristan had tricked the old nobleman into believing that Teodoro is his own missing son. By that time, however, Diana has learned that love does not respect position. She declares that they will marry anyway and keep the secret between themselves and Tristan. Federigo and Riccardo confess their plot to have Teodoro killed, and Diana gives Dorotea to Tristan as his bride. So all ends well, with honor saved and love triumphant. The gardener’s dog makes a final choice.