The Great War is over and the long-awaited engagement of Lady Mary and Matthew is on, but all is not tranquil at Downton Abbey as wrenching social changes, romantic intrigues, and personal crises grip the majestic English country estate. Wedding guests descend on Downton Abbey, where disasters - large and small - threaten. One is Cora's freewheeling American mother, who tries to loosen up her in-laws.