On a stormy night, a bounty hunter by the name of Halig arrives in Camelot. With him is a cage containing a beautiful druid girl. Gaius and Merlin, returning home from tending to a patient, spot the girl: Gaius explains that Uther offers a handsome reward for the capture of any who use magic. Despite Gaius's warnings not to interfere, Merlin returns later and frees the girl, though Halig soon notices she is gone. Merlin hides her in the caves beneath Camelot and promises to look after her. He introduces himself and she replies her name is Freya.
Halig goes to Uther and warns him of the druid girl's escape: Uther assigns a detachment of guards to search for her. In private, Halig warns him the girl is cursed, and that even the druids are said to fear her. Gaius is surprised to learn they even cast her out of their village, and Uther insists she be found immediately, especially after Halig mentions an accomplice may have helped her in her escape. A search for the pair swiftly ensues.
The next morning, Merlin manages to steal some food from Arthur, which he takes down to Freya. She asks him why he saved her from captivity, and he replies he sympathises with her, as it could have easily been him in the cage. As time passes, the pair begin to grow very close to each other. The next day, however, Merlin is leaving Arthur's chamber (having stolen more food from him for Freya) when Halig apprehends him. He doesn't believe Merlin's claims he doesn't know where Freya is, and is about to torture him when Arthur intervenes and says Merlin has his trust. Gaius also interrogates Merlin but he insists he had nothing to do with her escape.
That night however, at midnight, a strange, unseen creature prowls through the lower town of Camelot, attacking and killing two young lovers. The bodies are found the next day: Uther, Arthur and Gaius investigate the scene of the attack, but the evidence is confusing: the wounds on the bodies were clearly made by an animal, but human footprints lead away from the area. Uther swiftly concludes it to be the work of a magical creature. Merlin, unaware of this, continues to tend to Freya, and as time passes, the two begin to fall in love with each other. Eventually, Merlin decides he will help Freya not only escape Camelot, but stay with her. Though she insists her life is too dangerous for him to risk, she is clearly happy with the suggestion. Knowing she needs new clothes, Merlin manages to purloin a dress from Morgana's wardrobe, narrowly managing to fend off a surprised Gwen and an astounded Arthur, who clearly has his own ideas on what Merlin wants with the dress.
That night, the mysterious beast attacks again, this time killing two guards. Again, the evidence is the same: the victims are clearly those of an animal attack, but only human tracks lead away from the site. Arthur also remarks that eyewitnesses described the monster as a gigantic, winged black cat. Gaius, remembering Halig's warning, tells Merlin, who is deeply in love with Freya now, that she is cursed: every night at midnight, she transforms into a Bastet, a bloodthirsty and vicious beast. Horrified, Merlin confesses to freeing Freya, but refuses to believe Gaius's claim. Merlin begs for more time to get Freya to safety, but Gaius, knowing she will kill again, goes to tell Uther.
Merlin goes to the cave to warn Freya, only to find she has gone. Suddenly, the city bells toll midnight, and Freya, who was trying to escape Camelot, only to be caught by Arthur, Halig and a group of knights, transforms. The Bastet rips Halig apart and then attacks Arthur and his men. Arthur manages to badly wound the beast: as it retreats into the courtyard, pursued by Arthur and the knights, Merlin arrives. The Bastet gazes sorrowfully at Merlin: desperate to help Freya, Merlin uses magic to make a gargoyle topple at Arthur, distracting him long enough for her to escape.
Merlin goes back to the cave to find Freya, human again and dying from her wounds. She tearfully explains her curse was the doing of an angry sorceress after Freya accidentally killed her son in self-defence. Merlin carries her to a lake in the hope of healing her, but she is too badly hurt: however, she is grateful to him for making her feel loved again. With her last breath, Freya thanks Merlin and promises to one day repay his kindness, before she finally dies. A heartbroken Merlin places her body on a boat and uses his magic to give her a Viking funeral.
Back at Camelot, Gaius consoles Merlin, while Arthur, aware Merlin is upset, helps cheer him up with a bit of friendly banter, before setting him yet more tasks.