The play opens in the study at Hakon Werle's house during a dinner party for the return of Werle's son, Gregers, from the Hoidal mines. Gregers has not come home for fifteen years. Old Ekdal appears before two servants, begging to be let into the office. Ekdal was an army officer and partner to Werle until a forestry scandal sent him to prison over some scandal. He now works as one of Werle's copyists.
The dining room bursts open. Glancing dejectedly at Hialmar Ekdal, Werle remarks to Gregers in a low voice that it would seem no one noticed that there were thirteen at the table. Hialmar, who has overheard the conversation between father and son, remarks that Gregers should not have invited him. Hialmar tells of his life after his father's disgrace. To Gregers surprise, Werle helped him begin his photography studio and made it possible for him to marry. Coincidentally, Hialmar's wife, Gina, worked in Werle's house during the last year of Mrs. Werle's illness.
Suddenly Old Ekgal crosses the threshold. Werle gasps in disgust; Hialmar pretends to ignore his father. Old Ekdal exits apologetically, and the party resumes. A dejected Hialmar excuses himself. Gregers remains at the fireplace with his near-blind father. He asks how the Ekdals have come to ruin, accusing his father of passing the forestry scandal onto Old Ekdal. Gregers also recalls his father's interest in Gina; his mother revealed his betrayal to him on her deathbed. Werle rebukes him. He has plans to marry Mrs. Sorby and hopes his son will lend his approval. Gregers leaves the house, announcing enigmatically that he finally sees his mission in life.
Act II opens in Hialmar's studio. Hialmar reports on the party and declares his love for his family. Gregers appears at the door and chats with the Ekdals. When Gregers asks into Hialmar's daughter Hedvig, Hialmar reveals that she is going blind. Gregers wonders how such a sportsman like the Old Ekdal can live in such a stuffy town. Ekdal smilingly guides the party to the back of the room. A deep garret, filled with irregular nooks and crannies, appears through the doorway. Gregers observes a fowl lying in a basket—a wild duck that belongs to Hedvig. The household won the duck when Werle wounded it on a hunting expedition. Suddenly Gregers insists on taking the Ekdals' spare room. He will be like the wild duck himself.
It is morning in the studio. Hialmar retouches a photograph. Ekdal pokes in to ask if his son would like to join him in the garret. Eyeing Gina, Hialmar reluctantly stays at his desk. Hedvig enters and, seeing that her father yearns to join his father in the garret, offers to do his work.
Gregers appears at the door. He remarks how the duck looks different in the day. Hedvig enthusiastically agrees. Having left school to protect her eyes, she spends much of her time in the garret with the duck and the treasures of an old sea captain. Suddenly a shot rings out from the garret. Hedvig announces that the men are "out shooting." Hialmar replaces the gun on the bookcase.
Gregers asks if Hialmar leaves the studio work to his wife. Hialmar has dedicated himself to a great invention; his mission is to redeem the family's name. Hialmar cannot explain the invention just yet, since inventing takes time. Gregers wonders if the garret does not distract him. It seems Hialmar has, like the wild duck, dived into a "poisonous marsh." Gina and Hedvig bring lunch. At the same time, Relling and Molvik appear. Relling knows Gregers from the Hoidal works, where they had a few "skirmishes." The group toasts Ekdal and Relling cheers Hialmar and his family, as it is Hedvig's birthday tomorrow. Gregers notes that he does not thrive in "marsh vapors." Relling wonders if Gregers himself has brought the taint into the house with his "claim of the ideal."
At dusk, Gina has just taken a photograph of two sweethearts. A grim Hialmar returns from a walk with Gregers. He announces that he will take up the studio work himself on the morrow and pledges to never set into the garret again. Indeed, he wants to wring the wild duck's neck. Hialmar confronts Gina on her secret past. She was Werle's concubine.
A breathless Hedvig appears at the door; she has received a birthday gift from Werle. Werle has promised a monthly income to Ekdal that will pass onto Hedwig upon his death. Gregers warns that Werle is trying to buy him off. Hialmar tears the birthday letter in half and places it on the table. He confronts Gina and asks whether Hedwig is his. She replies that she cannot know.
Hialmar forsakes Hedvig and flees. Hedvig wonders why her father no longer wants her and thinks it is because she is not his child. Hedvig moans that Hialmar should almost love her more if so, just like the wild duck. Gregers suggests that Hedvig sacrifice the precious to prove her love for her father.
Cold morning light fills the studio; a snowstorm roars outside. Relling enters and reports that Hialmar is snoring on his sofa. He thinks Gregers is a sick, suffering from an "integrity fever" and a "delirium of hero-worship." Gregers asks what Relling prescribes as Hialmar's cure. Relling has given him the usual one: the Livslognen or "life-illusion" ("life-lie").
The door opens hesitantly, and an unkempt Hialmar enters. Gina asks if he is still leaving. Hialmar cannot live among traitors and he plans to take Ekdal with him as well. When Hedvig enters the room again, he repudiates her anew. Terrified, Hedvig takes the pistol from the shelf and creeps into the garret.
Exhausted, Hialmar slumps onto the sofa and unthinkingly begins to eat his breakfast. Gina suggests that he stay in the sitting room for a few days. Hialmar agrees. He also glues Werle's letter back together. It is for his father, or him, to decline the offer.
Suddenly a shot rings out from the garret. Gregers triumphantly announces that Hedvig has sacrificed her duck to prove her love for her father. Hialmar tears open the garret door, only to discover a prostrate Hedwig. Relling declares the child dead: the bullet has pierced her chest. Once the members of the household have retired, Relling informs Gregers that Hedvig has certainly killed herself.