Hostage to Pleasure is Dorian's story. Dorian is the sentinel that cannot change into leopard form. His match is Ashaya Aliene the Psy that we've been slowly introduced throughout the previous two books. She was involved in Protocol I because the Psy held her son hostage. In Mine to Possess she helps Tally find Jon and also helps Noor escape from the lab in exchange for one day helping her get her son out. 
 Hostage to Pleasure opens with the DarkRiver changelings getting Keenan away from his captors. At the same time Ashaya has a plan to escape from the lab. With help from other rebel psy she is successful, but she also has her own plans that involve exposing the work the Psy is doing to the whole population. 
 At this point pretty much the series follows its particular pattern. This one is no exception. It's still a pretty good series. But it's one that I have to take a break in between books otherwise I feel it will be too much of the same. Each book does progress on the war between the Psy and the changelins, but the developments in each book on add a small amount of information. I'm really waiting for some kind of big reveal. I guess the only way to get that would be to keep reading!