Act 1
Zoe, a queen who possesses the gift of turning into a swan by means of a veil, is unhappily trapped in her marriage to her husband, King Zeno. Only once a year is she permitted to take the form of a swan, flying to a mystical lake whose waters confer eternal youth and bathing in it in human form. At this “Swan Lake,” one can let oneself go, live free of social norms and constraints, and simply be oneself. One day Zoe meets a stranger named Benno. The two fall in love and come closer. When King Zeno catches the couple, he has Benno arrested. However, Theo, the King's personal physician, helps the prisoner to escape, revealing to him that he may have a chance of seeing Zoe again at the distant Swan Lake. In this hope, Benno embarks on a journey to the Swan Lake and waits on its shores for Zoe. There the two are in fact reunited and give rein to their love together for the first time. After Zoe's return to the court, King Zeno punishes his wife by destroying her magic veil and thus cutting off her wings, after which Benno waits in vain for years for the return of his beloved.

Act 2
Around 20 years later: The recently widowed Queen Zoe must now accept another loss: having reached adulthood, her son, Prince Kallisto, is compelled to go to war. On the very shores of the Swan Lake, he is wounded and then found and nursed back to health by the aged Benno, who takes him in like a son. Benno tells his protégé about the liberating nature of the lake, which is visited each year by swan people. He then dies in Callisto’s arms. Now living as a hermit himself, Prince Kallisto seems to court good fortune one day when he secretly watches some of the swan people swimming in the enchanted lake. Kallisto manages to get hold of the veil of the beautiful Odette, committing her to himself, since having lost her wings, the princess must now seek shelter on the shores of the Swan Lake. She thus willingly engages with the Prince, who pretends to be her rescuer and to take her in benevolently …

Swan Lake is generally presented in either four acts, four scenes (primarily outside Russia and Eastern Europe) or three acts, four scenes (primarily in Russia and Eastern Europe). The biggest difference of productions all over the world is that the ending, originally tragic, is now sometimes altered to a happy ending.

Prologue
Some productions include a prologue that shows how Odette first meets Rothbart, who turns Odette into a swan.

Act 1

A magnificent park before a palace

[Scène: Allegro giusto] Prince Siegfried is celebrating his birthday with his tutor, friends and peasants [Waltz]. The revelries are interrupted by Siegfried's mother, the Queen [Scène: Allegro moderato], who is concerned about her son's carefree lifestyle. She tells him that he must choose a bride at the royal ball the following evening (some productions include the presentation of some possible candidates). Siegfried is upset that he cannot marry for love. His friend Benno and the tutor try to lift his troubled mood. As evening falls [Sujet], Benno sees a flock of swans flying overhead and suggests they go on a hunt [Finale I]. Siegfried and his friends take their crossbows and set off in pursuit of the swans.

Act 2

A lakeside clearing in a forest by the ruins of a chapel. A moonlit night.

The "Valse des cygnes" from act 2 of the Ivanov/Petipa edition of Swan Lake
Siegfried has become separated from his friends. He arrives at the lakeside clearing, just as a flock of swans land [Scène. Moderato]. He aims his crossbow [Scène. Allegro moderato], but freezes when one of them transforms into a beautiful maiden, Odette [Scène. Moderato]. At first, she is terrified of Siegfried. When he promises not to harm her, she explains she and her companions are victims of a spell cast by the evil owl-like sorcerer Rothbart. By day they are turned into swans and only at night, by the side of the enchanted lake – created from the tears of Odette's mother – do they return to human form. The spell can only be broken if one who has never loved before swears to love Odette forever. Rothbart suddenly appears [Scène. Allegro vivo]. Siegfried threatens to kill him but Odette intercedes – if Rothbart dies before the spell is broken, it can never be undone.

As Rothbart disappears, the swan maidens fill the clearing [Scène: Allegro, Moderato assai quasi andante]. Siegfried breaks his crossbow, and sets about winning Odette's trust as the two fall in love. But as dawn arrives, the evil spell draws Odette and her companions back to the lake and they are turned into swans again.

Act 3

An opulent hall in the palace

Guests arrive at the palace for a costume ball. Six princesses are presented to the prince [Entrance of the Guests and Waltz], as candidates for marriage. Rothbart arrives in disguise [Scène: Allegro, Allegro giusto] with his daughter, Odile, who is transformed to look like Odette. Though the princesses try to attract the prince with their dances [Pas de six], Siegfried has eyes only for Odile. [Scène: Allegro, Tempo di valse, Allegro vivo] Odette appears (usually at the castle window) and attempts to warn Siegfried, but he does not see her. He then proclaims to the court that he will marry "Odette" (Odile) before Rothbart shows him a magical vision of Odette. Grief-stricken and realizing his mistake, Siegfried hurries back to the lake.

Act 4

By the lakeside

Odette is distraught. The swan-maidens try to comfort her. Siegfried returns to the lake and makes a passionate apology. She forgives him, but his betrayal cannot be undone. Rather than remain a swan forever, Odette chooses to die. Siegfried chooses to die with her and they leap into the lake. This breaks Rothbart's spell over the swan maidens, causing him to lose his power over them and he dies. In an apotheosis, the swan maidens watch as Siegfried and Odette ascend into the Heavens together, forever united in love.