This is the story with Lady Christina de Souza, played by Michelle Ryan, set after "Planet of the Dead" and, not stated outright but implied by the story, after "The Waters of Mars". As I've stated before, I don't like Lady Christina, so the fact that I thoroughly enjoyed this story despite her being in it should tell you something.
If you really boil the story down to its basic elements, it sounds pretty boring. There's this alien named the Deagle (I can't find a definitive spelling of his name, so that's what you're getting) who thinks he's the richest, most powerful, most entitled creature in the universe. He's decided his new hobby is flying through time, capturing the last members of endangered species and killing them himself. Thus, the Doctor is racing him, trying to capture those animals before the Deagle can and put them into protection so that they can flourish again. While he's chasing after the last two black rhinos, he bumps into Lady Christina, who is there to steal an expensive piece of jewelry off the poacher who was about to kill them.
As I noted, the story seems pretty basic. What really shines here is how the story is told. The style is rather Douglas Adams - the way that the characters' histories and current intentions are described reminded me of Dirk Gently's Holistic Detective Agency. One of the lines I clearly remember went something like this: "Safety was one of the things that the Doctor didn't think much about. He viewed safety in the way most of us view cannibalism: We all are aware it exists somewhere, but aren't particularly interested in trying it." Jacob Dudman did a great job relating the surrealism in a deadpan way. Then, layered on top of that are the twists that make the story interesting and fun. This gave this Tenth Doctor story a classic feel.
The one weakness of the audio was Lady Christina herself, and it's not that Michelle Ryan did a bad job. I feel that the character wasn't depicted well. What I remember about the character from "Planet of the Dead" was that she was a confident, skilled thrillseeker without much of a moral compass. When the bus landed on the alien planet, she took charge of the group, to the consternation of the Doctor. What I didn't like about her was her lack of morality and compassion for others - it's just not the type of character I like (see River). In this audio, these traits were mentioned (she helped foil the poacher's plan to kill the black rhino because she wanted to steal the jewelry, not to stop the killing, for example, and later on, she uses her rappelling skills to break into the cages where the animals were being held), but for the most part, she simply whined: Why are we doing this like this? You're really going to do that? etc. etc. Not at all the Lady Christina that I remember.
One last important: the story was set in a delicate part of the Tenth Doctor's existence, and it handled that well, too, coupled with Jacob Dudman's skilled portrayal of the Doctor. All in all, it came together as a great audio and ended the Tenth Doctor Chronicles on a high note.