In the Darth Vader's Secret Apprentice Novel, the plot is centered around the eponymous character. The apprentice is trained by Darth Vader in the ways of the Sith. He gets involved in galactic politics and power struggles from the Sith perspective. As he progresses in his training, he uncovers dark secrets about the Sith order and Vader's own past. His journey is filled with danger as he battles Jedi and other opponents who stand in the way of the Sith's grand plans. He also has to deal with his own moral compass being warped by the teachings of the Sith, which makes for a very interesting character arc.
Well, in Darth Vader's Secret Apprentice Novel, the story focuses on this secret apprentice. He has to prove himself worthy in Vader's eyes. There are battles against the Jedi and other enemies. The apprentice also discovers secrets about the Force that are not known to many. He is constantly in a struggle to become a powerful Sith while also dealing with internal conflicts within himself and his loyalty to Vader.
The main plot revolves around Darth Vader's secret apprentice. He undertakes various missions and faces numerous challenges in the dark side of the Force universe. His relationship with Vader is complex, full of training, tests, and the pursuit of power within the Sith hierarchy.