“Power of the Dark Crystal” is set many years after the events of the first movie, and Jen and Kira are now king and queen. When the Gelflings welcome a fiery and an unexpected visitor to their castle, they learn that the very existence of the U-mun people, who live in the searing heat in the center of the earth, is threatened by the death of their sun. There is apparently only one thing powerful enough to heal the dying sun – a shard of the Crystal. The Gelflings refuse the U-mun request for a crystal shard, as breaking the crystal would disturb the peaceful surface world’s balance. However the desperate visitor takes matters into her own hands and steals a shard, making a hasty escape through tunnels deep in the bowels of the castle. The surface world once again falls into darkness – as the UrSkek split into Mystics and Skeksis. So begins the adventure of Thurma, a girl “made of fire”, who bands together with Kensho, an outcast Gelfling, in an adventure in the “inner-world” over the fate of the planet. Directed by Clone Wars creator Genndy Tartakovsky this should be a worthy sequel to the first! 
From the Muppet Wiki:
