In only the second multi-issue story arc to appear in Fantastic Four (the first was a confrontation with the Frightful Four), Stan Lee and Jack Kirby really hit their stride.
The issue opens with a battle between the Fantastic Four and Black Bolt (first seen in the final panel of the previous issue). We see that Black Bolt is indeed formidable. As far as sheer power goes, he may be one of the most powerful beings they've yet encountered - able to go toe-to-toe with the Thing!
Black Bolt leads a group of self-described 'Inhumans', which includes Crystal, Gorgon, Karnak, Medusa and Lockjaw (all of whom we met last issue), as well as a merman type character called Triton - who proves to be more than a match for the Human Torch!
Meanwhile, the Inhumans are being pursued by one known as the Seeker who breaks into the Fantastic Four's headquarters and kidnaps Dragon Man.
The issue is great for a number of reasons. It's the first real introduction of Black Bolt (since we only saw him for one panel last issue and he didn't do anything), it reveals the origin of the Inhumans, and is chock full of Kirby action like the above panel from the battle between the Thing and Black Bolt.
Sure, it's a little intellectually light, but it sure is fun!