Ep 29. Magnetic Attraction

Lotor's newest Robeast generates an enormous cloud of fire and heat, vaporizing all the water around the Castle of Lions, including the entire moat. The Voltron Force is interrupted from their martial-arts training, and head off to battle. Princess Allura can't seem to activate the Blue Lion. Her father's ghost reveals the secret of the Lions: They are powered by the elements of Arus. Without any water, the Princess has no way of operating the Blue Lion, and the team can't form Voltron.