Merlin stumbles across a secret chamber, which is home to magical books and strange fantastical objects. He literally stumbles over a small chest and hears a noise coming from inside. Intrigued, he tries to open it, but to no avail. His eyes flash as he incants a spell and, to his surprise, out springs a Goblin. The Goblin escapes from the secret chamber, leaving a trail of destruction in his wake. Merlin chases after him, terrified of the consequences.
Uther summons Gaius to see him. When he arrives, he is shocked to find a completely bald King. Gaius quickly realises this is the work of an enchantment and the Goblin must be to blame. He warns Merlin that they must catch it before any real damage is done.
Using gold coins, Merlin and Gaius lure the Goblin into a trap. But the increasingly sneaky Goblin turns into a magical ball and, while Merlin is distracted, inside Gaius' ear. The King's physician has been possessed, and ransacks his own chambers. Merlin wonders why Gaius is acting so strangely.
Morgana asks Gaius for a sleeping draft and is taken aback by the curt response. Gaius bristles at being put in his place and tells her he knows she would see Uther dead and Camelot destroyed. With his back turned, he incants a spell and hands Morgana a vial of purple liquid. Morgana leaves, fearing Gaius is on to her.
Merlin realises Gaius is the Goblin when he catches him slobbering over some gold. Merlin confronts him, wanting to know what he has done with Gaius. The young warlock is assured Gaius is still in there... somewhere.
During a second confrontation with Gaius, Merlin uses magic to trap him in the room. Gaius eyes Merlin with renewed interest, but says it changes nothing. As long as he remains in Gaius' body, Merlin can't hurt him. Merlin vows to force the Goblin out, once and for all.
To Merlin's amazement, Arthur arrives with orders to arrest him. In fear for his life, Merlin exclaims Gaius has been possessed by a Goblin. Arthur is willing to hear him out, but without proof, not even the Prince can save him. Merlin is sentenced to death.
Alone and terrified, Merlin is left to await his fate in jail. The young warlock is given a break when the guard falls asleep. Merlin uses magic to aide his escape. Risking it all, Merlin goes to see Gwen. They come up with a plan to save Gaius, but Arthur will need to be onboard.
Gwen summons the courage to face Arthur. When she arrives at his chambers, she is shocked to find him sitting on the floor, with the ears of a donkey. She desperately tries to find out what has happened to him, but he is unable to speak; he can only bray like a donkey.
Gaius slobbers over the coins in his chest. Merlin and Gwen emerge from their hiding place and he quickly realises his coins have been poisoned. Gaius slumps to the floor, as a ball shoots out of his ear. Gwen is about to administer the antidote, but the ball flies straight at her. She falls backwards and the antidote gets lost in a mix of similar vials, sealing Gaius' fate.
The sneaky Goblin tries to possess the young warlock. Without thinking, Merlin opens his mouth in shock. Before he can close it, the ball shoots inside. Merlin freezes in horror. He dives for the Goblin's chest and spits the ball inside, trapping the Goblin for good.
There is no time for celebrations as Merlin's attention quickly turns to a dying Gaius. He joins Gwen who is frantically looking for the antidote. Merlin picks up a vial that looks like the right one and pours it down Gaius' throat. Nothing happens for what seems like an age and then...Gaius opens his eyes.
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