The "critical threshold" of this novel's title refers to the minimum level of acceptibility, in terms of environmental and ecological conditions, that would make an alien world suitable for human colonization. And in their second adventure, the stalwart crew of the Daedelus, the craft sent from Earth to re-establish contact with lost and perhaps failed colonies, lands upon the planet Dendra. Dendra seems an ideal world in many ways: the weather's always great, and the entire planet is ringed by a single continent covered over by a massive forest teeming with plenty of edible flora and fauna.
But even before the Daedelus lands, it becomes very clear that Dendra is a world that has fallen below said critical threshold. The colonists number fewer than a hundred after a century and a half, and not only are they gaunt and sickly-looking and appear even a bit mad, they're still wearing the tattered rags of the 150-year-old uniforms the original settlers wore! No one on the Daedelus crew can find any trace of the original settlers' databanks, and, most curiously, there is a wall seven miles long that surrounds the puny area where the survivors live. But is the wall to keep something frightening in the forests out, or the colonists in? Biologist Alexis Alexander theorizes the latter. Something is rotten on Dendra. What could it be?
Stableford sets up a nifty mystery right at the outset that keeps you compelled all through this book, which is as short and sweet as The Florians. Happily, you don't have to have read The Florians to understand this one. Like that book, the narrative follows a reliable solve-the-problem formula hard SF nuts will go for. And like that book, it's awfully talky. But the mystery is eerie and intriguing and Stableford writes dialogue (even lengthy speeches) well. No one sounds dated.
There is less action in Critical Threshold than in The Florians. Much of the novel is taken up by a lengthy, travelogue-like exploration sequence in which Alexander and two crewmates journey deep into the forest. But though nobody is pounced upon by a ravening beastie, this sequence allows Stableford's hard SF world-building skills to shine. It's truly impressive how he has realized the alien environments the Daedelus visits right down to the fine details of their ecosystems and the evolutionary paths of their animal life. If Critical Threshold were any longer than it is, the book might be a bore. But at 160 pages, passages like this are easily enjoyed.
There is also less interaction with the pathetic natives this time, and that admirably keeps the book from being a retread. That's always a risk in an episodic series like this one, the structure of which is reminiscent of an above-average SF television show. Those of you looking for some good, old-school "people on a ship" space adventuring would do well by the Deadelus novels. I'd like to see more authors today writing simple little old-fashioned tales such as these.