This so-called "Cosmic Size-Special" is pretty much what used to be called a fill-in issue, although the story here is almost twice as large as normal, and there's no clear indication of a cliffhanger halfway through, so it couldn't have been a two-part fill-in. But it feels like one of those classic fill-in issues, nevertheless, giving us a story which explores the nature of the characters without changing the status quo in any way by the end of the story.
Yet such a story is not necessarily a bad thing, and in the case of Cary Bates and Bing Cansino's "Hothouse," a story which explores the nature of the Fantastic Four seems like a good idea. Bates, an industry veteran who, after a long absence, returned to superhero comics in 2008 with his "True Believers" series, provides an excellent story here, and Cansino, whose work is unfamiliar to me, provides solid pencils and inks.
This is a 64-page Special, but it's 40 pages of new material ("Hothouse") and 24 pages of reprints (the John Byrne Spinnerette story from "Fantastic Four" #237), and that 64-page count includes the ads, if you're wondering how much material you get for your five bucks.
The John Byrne reprint seems randomly chosen, although it perhaps has a tenuous connection with the main "Hothouse" story in this issue. Byrne's story, which is not the best example of his superior work from his 1980s run on the series, deals with an alien life form, shows a bit about the soap opera dynamics within the Richards family, and gives us a bit of Franklin Richards. Those things do feature in "Hothouse," and maybe that's all the connection we need, since there's no in-story reason for the Byrne reprint to appear. I don't know if people really buy these kinds of Specials (or avoid them) based on the reprints in the back, but I can't imagine that this particular Byrne story is much of a selling point.
"Hothouse," though, is a good story all by itself. It tells of a quarantine in the Baxter Building, as an exploration into a pocket dimension has led to a kind of alien bacteria hiding out in the Thing's crevices. But there's a more sinister agent at work in the story, and as the house-bound heroes (and, importantly to the story, Franklin and Valeria Richards) begin turning on each other. It's a classic sci-fi set-up, with each character becoming increasingly paranoid and vengeful, but Bates provides excellent characterization and allows the members of the team to express their true thoughts about one another. There's nothing particularly surprising here, but Bates writes all of the characters well, and paces the story perfectly. It's far from the "Cosmic" implied in the title, as it's a very inward, twisted family melodrama, but it's a good one.