S1·E4 Lady of the Lake

Whilst Guinevere wants to leave Camelot with Leontes, he's obliged to stay as one of Arthur's knights being trained by brilliant strategist Gawain, who breaks Arthur's sword in combat. Having warned Arthur to resist his feelings for Guinevere, Merlin sets out to have a new sword made by reclusive bladesmith Caliburn, a widower living with his daughter.