This is the 4th novel of the "The Fall"-miniseries. The race to catch the culprit for Bacco's assassination continues. The Titan is recalled to Earth where Riker's promoted to Admiral. He's wondering about the reason for that when he receives a transmission from Picard who tells him that the Tzenkethi aren't responsible for the assassination, that it was Cardassians. So Riker starts snooping since the Ishan-administration still maintains the culpability of the Typhon Pact. And he sends Vale on a mission to find out the truth about what happened with Bashir and the Andorians. Meanwhile, Tuvok is recruited for a covert operation to capture the assassins, together with Nog and Tom Riker.
So, the story is pretty much divided into 4 plotthreads. All of them advance the plot surrounding the assassination, but unlike the other 3 novels of the Fall, this one isn't really a stand-alone novel. You definitely need to have read the "Revelations..." and "A Ceremony of Losses". 2 of the plotthreads, namely Vale's and Troi's are about uncovering the Andor-story... which is interesting in and of itself because the Ishan-administration managed to practically bury Bashir in a secret facility, just to shut him up. And they aren't really willing to listen to the Andorians, either. So, no one except for those directly involved (meaning Ishan, Bashir, Dax) really knows what happens, and what the administration did and knew. Therefore while it is important to take those steps within the narrative to uncover the conspiracy, for someone who read "Ceremony", who therefore knows what happened, these parts of the story are a bit repetitive, despite questions of loyalty by Vale's temporary crew etc.
That leaves Tuvok (and Nog and Tom Riker... I still don't really understand why he had to be included, to be honest) being part of a mission that ever gets more immoral when the perpetrators are delivered to a Klingon torture base instead of to Earth so that they can stand trial. Will Riker finds out about the mission and who ordered it, and eventually comes to the rescue... unfortunately, all evidence pointing towards Ishan is lost. The mission is quite straight-forward and predictable, that leaves Riker's part as the only actual plotthread holding some suspense because why was he promoted? And who's spying on him?
I think the last question is the most interesting one because as it turns out, Ishan apparently has a wide network of operatives who are willing to do anything, moral or immoral, to further their cause. And he himself as a Bajoran is willing to ally himself with isolationist Cardassian splinter groups to get rid of enemies. So, while the Fall is a pretty engaging mini-series so far, the Poisoned Chalice itself didn't impress me as much as its predecessors, unfortunately, because it suffers a bit from the "penultimate part of series"-syndrome, paving the way for the conclusion, but not actually leading any of the plotthreads there itself. Which is a bit frustrating perhaps.