Love. As seductive and dangerous as magic, this potent, unpredictable force proves to be the ultimate weapon in the epic battle between good and evil in ABC Studios' "Once Upon a Time: The Complete Fourth Season". Soon after Emma and Hook unwittingly bring Elsa of Arendelle to Storybrooke, they encounter the mysterious Snow Queen, whose relentless obsession with both Emma and Elsa has chilling consequences. Meanwhile, it appears Regina has at last found true love with Robin Hood, but how will she react when her happy ending is thwarted yet again by a maddening twist of fate? Then, a banished Rumplestiltskin enlists the Queens of Darkness (Maleficent, Ursula, Cruella De Vil) in an insidious scheme to rewrite their own stories and corrupt the Saviour by turning her newly blissful heart pitch-black.