Seems pretty straightforward to me. (Spoiler warning!)
tangles with some Tressaurians, a reptilian race the Captain encountered many years ago. These nasty lizardfolk have stolen a Tholian device that is capable of shifting objects into another dimension. When Exeter
follows the Tholians to their home base, they discover a similar device millions of times more powerful that is using a star for a fuel source. This device is capable of shifting whole sectors of space into another dimension.
And that's where we left off two years ago.