Two of the FF's old enemies--the Puppet Master and the Mad Thinker--team up to get revenge on our heroes. Thinker figures out the exact amount of radioactive clay Puppet Master needs to make a puppet of Professor X and gain control over him. Then Professor X is forced to order the X-Men to attack the Fantastic Four.
Nowadays, the X-Men are so perpetually popular its easy to forget there was a time when they were second-tier characters in the Marvel Universe. But The X-Men was never that big a seller in the 1960s. Cross-overs like this (or like the Human Torch/Iceman team-up in Strange Tales a few issues back) are quite obviously meant to generate interest in the X-Men's own book.
And that's just fine, as long at the story is good. And this one is good. Within the context of a comic book universe, the Thinker's plan makes sense. And, as usual, the action sequences are marvelous. There's a half-page panel near the end of the story, when all the heroes have figured out what the heck is going on, that features them going up against the Thinker's Awesome Android. That one panel alone would have been worth the price of the book. An earlier scene in which Iceman encases Ben's head in a big blog of ice--then adds a couple of blocks of ice to Ben's feet--is also pretty cool to look at.
There's an interesting and harmless mistake near the beginning of the story. Reed and Sue are reading a newspaper account of the X-Men's activities and Sue reads off a list of their opponents. She includes "the Space Phantom" on the list. But the Phantom had fought the Avengers, not the X-Men. Oh, well, poor Stan Lee was juggling so many characters at this point that I think we can forgive him if he loses track of a few details.
It all ends with the Enforcers captured. The Hulk vanishes back into the caves and the Goblin escapes. The movie is cancelled and Peter has to take the bus back home.