WHEN SHE LEARNS KING ARTHUR MAY BE HIDING HIS TRUE INTENTIONS, MARY MARGARET MUST CONVINCE DAVID AND THE HEROES OF HER SUSPICION BEFORE THEY JEOPARDIZE EMMA'S SAFETY, ON ABC'S "ONCE UPON A TIME"

Sinqua Walls Guest Stars as Lancelot, Joana Metrass Guest Stars as Guinevere and Liam Garrigan Guest Stars as King Arthur

"The Broken Kingdom" - After receiving a cryptic warning from Lancelot about Arthur's intentions, Mary Margaret realizes Arthur may be the heroes' biggest threat, but when she is unable to convince David of the danger, she takes matters into her own hands. Meanwhile, Hook's unwavering love for Emma provides a glimmer of hope in her struggle against the unrelenting voice of Rumplestiltskin. In a Camelot flashback, Guinevere senses that Arthur is losing his way, consumed by his obsession with making Excalibur whole, so she sets out with Lancelot on her own quest into the heart of darkness. In Storybrooke, Dark Emma unleashes a secret weapon in the next phase of her plan to find the brave soul she needs to draw Excalibur from the stone, on "Once Upon a Time," SUNDAY, OCTOBER 18 (8:00-9:00 p.m., ET) on the ABC Television Network.

"Once Upon a Time" stars Ginnifer Goodwin as Snow White/Mary Margaret, Jennifer Morrison as Emma Swan, Lana Parrilla as the Evil Queen/Regina, Josh Dallas as Prince Charming/David, Emilie de Ravin as Belle, Colin O'Donoghue as Hook, Jared S. Gilmore as Henry Mills, Sean Maguire as Robin Hood, Rebecca Mader as the Wicked Witch/Zelena and Robert Carlyle as Rumplestiltskin/Mr. Gold.

Guest stars include Sinqua Walls as Lancelot, Amy Manson as Merida, Liam Garrigan as King Arthur, Andrew Jenkins as Percival, Joana Metrass as Guinevere, Olivia Steele Falconer as Violet, Webb Hayes as Young Arthur, Ehren Kassam as Young Kay and Dalila Bela as Young Guinevere.