The Surprise Quest - S1.E4

Arthur's loyal magician, Merlin, has discovered a potion which makes the person you give it to fall head-over-heals in love with you. Arthur plans to catapult the potion at Guinevere so she'll finally marry him, but he accidentally hits her chaperon, Petal. He asks Merlin for an antidote, but is told that a few special ingredients are needed, a feather from the confused back wards flying owl and a single hair from the maine of the invisible unicorn, so Arthur sets out to find these whilst trying to avoid Petal who now wants to marry the King.