The Enterprise is headed to Vega III to sign a trade treaty, to get Korium. The way the book phrases it sounds a bit ominous, though:
When they arrive, they are told they must leave or be destroyed. Kirk is friends with Queen Vanadala, though, and knows something must be wrong, so Kirk, Spock, and McCoy beam down to investigate.
On the planet, they find that Klingons have taken over Vega III and imprisoned Queen Vanadala in order to take the valuable Korium for themselves. Kirk, Spock, and McCoy knock out three Klingon guards and steal their clothing in order to observe undetected (knowing, as we all do, that Augment Klingons are indistinguishable from humans).
They meet the Klingon leader, and Spock recognizes him:
Our heroes are captured and imprisoned with Queen Vanadala, then all four are taken out to be executed. However, Kirk comes up with a cunning plan: stall for a few minutes until the time they have arranged for Scotty to beam them all back up, so they will be saved.
He does and they are.
Like The Truth Machine, it's not much of a story, but it's a picture book, so... good enough.