The Hunter's Heart (S04E11)

Morgana finds a powerful ally in the Southron warlord Helios. Together they hatch the perfect plan to force Camelot to its knees. With the kingdom - and Arthur - preoccupied by the arrival of the beautiful Princess Mithian, it falls to an absent friend to raise the alarm: Guinevere. But with past wounds still raw, can love really conquer all? Or is a deadly arrow destined to fly straight into the heart of Camelot? (BBC)