While Merlin and Arthur are hunting in the woods one day, they hear heart-piercing shrieks coming from a nearby village. They see the townsfolk leading an old woman to death at the stake for the crime of sorcery. But Arthur, who ascertains that she had not been given a fair trial, takes the woman away. She feels her end coming soon, and offers Arthur a gift in return for his kindness: the Horn of Cathbad, as Gaius later explains, which was used by high priestesses to travel into the spirit world.
Gaius warns Arthur of the danger of using the horn, but Arthur longs too strongly to see his father once again, so he and Merlin travel to a sacred place to pierce the veil between worlds. On the other side, Arthur sees Uther, who is disappointed in his son's rule - making commoners into knights, and marrying a serving girl instead of forming an allegiance by marrying a royal daughter. Arthur tries to defend himself, but slinks away in shame, stealing a final glance as he returns to the land of flesh and blood.
The following day, as the court convenes around the round table, the doors to the great hall blow open, and the chandelier crashes on top of the table. Later, as Percival is changing out of his armor, the arms start banging and clacking of their own accord, and a battleaxe gets lodged in Percival's shoulder. Speaking to Gaius, he says he felt someone there with him, a sense which Merlin later tells Gaius he felt, too. Gaius knows something is afoot, and when Merlin admits that Arthur opened the veil between worlds, Gaius knows that Arthur's backward glance allowed Uther passage into the world of the living.
Arthur is loathe to believe it could be his father's ghost, but when Gwen is left to die in a burning kitchen, with no one around, Arthur knows he must act. Gaius prepares a concoction for the two boys to drink, allowing them to see Uther in his corporeal form. Since Arthur let him free, only he can use the Horn to return him to the world again.
But when his chance comes, Arthur's love for his father overcomes him. He tries to explain why he rules the way he does, and finally proclaims that he will never rule the way Uther did, but inspire with love, rather than control with fear. Uther grows stormy at his son's truculence and slings a shield at his head, knocking him out cold.
Just then, Merlin appears and draws Uther's ghost from Arthur's prone body before it's too late. Uther sends a bench flying at Merlin, but the wizard blocks it with a glance, revealing his true identity. Uther's rage strengthens, and a battle begins in the halls of Camelot that ends with Uther pinning Merlin to a door with spears. He menaces Merlin as he raises his sword, but Arthur appears and blows the horn. Uther tries to warn his son about Merlin, but his voice dissipates as the veil closes.