Meet Dr. Doom
This issue is significant because it introduces perhaps the greatest villain in the Marvel Universe, Dr. Doom.
As a story goes... well it just isn't that strong. Like most issues of Fantastic Four, the first half is better than the second. Probably the only thing I don't like in the first few chapters (other than standard corny Silver Age dialog) is the fact that Johnny Storm is reading a Hulk comic and compares the Hulk to the Thing - O.K., he read a Sub-Mariner comic last issue, I can kind of excuse that since Namor is supposed to have been (in the Marvel Universe) a historical figure, so theoretically, there could be a comic about him. But they were promoting the heck out of The Hulk in margin ads in the last issue, so this seems to be just a way to cross-promote that book. It irks me because it makes for stories where the Marvel Universe characters are themselves comic characters in their own universe. Of course, this is before Marvel did a lot of crossovers, so perhaps in this nascent universe they hadn't yet decided that the heroes all existed in the same universe - but if they did, all one member of the Fantastic Four (or Doc Ock) would have to do to learn Spider-Man's secret identity would be to... pick up a Spider-Man comic!
Anyway, that rant aside, the first half of the comic is pretty good. It starts with an evil genius named Dr. Doom, who captures the FF and then forces them to do his bidding. But, that's where it goes off the rails, in order to get them to comply Doom takes Invisible Girl hostage (pretty much taking her offstage for most of the rest of the comic) and then sends the remaining three to the past to recover Blackbeard's treasure. Once in the past, the three need to garb themselves to fit in... and what do you know? The first thing they encounter is some pirates who have stolen a big sackful of clothes. What a coincidence!
To make a long story short, they are Shanghaied aboard a pirate ship, become pirates - the Thing becomes Blackbeard himself, because the sackful of clothes happened to contain a fake black beard in it - of course. Later the thing orders his crew to take Johnny prisoner, and not to worry about his powers, because he is still 'soaking wet' - although, when that happens, who knows?
Finally Richards double crosses Doom by taking back a chest full of chains, stating that "we never promised to bring the treasure itself", just the treasure chest - except that that is wrong - they DID promise to bring the treasure itself, if you page back and look.
Johnny also uses his powers to get them through a moat by boiling a section off and fusing the ground to glass - which really makes no sense since the water would flow back over that section... but whatever.
Ultimately, Dr. Doom feels cheated at the end - and who can blame him, after reading this story, I do too!