Here is a brief plot summary for Power of the Dark Crystal pieced together from the information provided in press releases, interviews and other official sources.
Spoiler Warning: This post contains spoilers about the plot of the film. If you don't want to be spoiled, please do not read on.
Power of the Dark Crystal is set many years after the events of the first movie. Jen and Kira, the gelflings, are older and are reigning as king and queen in the Castle of the Crystal. After the Gelflings welcome a fiery. and 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 planet, 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 request for a crystal shard, as breaking the crystal would disturb the peaceful surface world's balance. But 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.