Foundation and Empire, by Isaac Asimov (pub. 1952)
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Caution: potential spoilers.
The first part of the book (The General, comprising about a third of its total length, the second part being twice as long as the first) is about a general named Bel Riose, the last great general of the fading Galactic Empire (around 195 F.E. It continues to be difficult to follow the timeline of the books, as specific periods of time between specific events are rarely given, much less specific dates. And of course, I didn't jot down any temporal references while reading, so I'm skimming a bit while writing my reviews, which means I likely miss some things. But... in the first book, Hober Mallow had visited a couple of worlds, the more important being Korell; however, he also met a man named Onum Barr on Siwenna. About 40 years after that, Bel Riose was on Siwenna, where he met Ducem Barr, Onum's son. Hence, 195 F.E.)
Bel Riose has heard of the Foundation, which is by now taking on a sort of legendary status, bordering on mythical. And he's heard of Seldon's Plan. And he wants to try to defeat the Foundation (so there's the fourth Crisis). He forces Ducem Barr to provide him with some assistance, though Barr is a firm believer in the Seldon Plan, and therefore, the undefeatability of the Foundation. And, as it turns out, he's right. And I can't really think of more to say about this story.
The second, and far more interesting part of the book (The Mule), is about a mutant who calls himself "the Mule," who has been quickly building his own little Empire, headquartered on the planet Kalgan. (It's set about 100 years after the previous Crisis. I'm not clear on the exact timing, but Wikipedia says around 310 F.E., which I doubt is right. A shame, because after the discrepancy between my reckoning and Wikipedia's concerning the third Crisis, it seemed we were back in sync for the fourth. However, in this story, a character named Bayta says two things: the story of Seldon's Plan has been known by the Foundation for almost three centuries- which could mean since the founding, but more likely means since the first Crisis, when the truth was fully revealed, which makes the timing even more suspect. The other thing she says is that it's been almost a century since the last Crisis, which if the date of 195 F.E. is accurate, would mean the current year must be less than 295 F.E. Sigh.) Anyway, because of the Mule's mutant powers (the nature of which I won't reveal), his actions were unpredictable by Seldon, and therefore throw the whole Plan out of whack.
I'm getting ahead of myself, however... There was an impending Crisis, one of internal conflict within the Foundation, between the increasingly corrupt Mayoralty, and the Independent Traders. There is a psychologist on the Foundation named Ebling Mis, who has been studying the Time Vault where Seldon's hologram appeared just before the first Crisis and just after the second. For the third and fourth Crises, no one had been in the Time Vault, but Mis had reason to believe the hologram had appeared with a message each time, and he could predict when Seldon would make his next appearance, which was four months hence. This time a number of individuals were there... and while they thought (or hoped) Seldon would mention the Mule, instead he mentioned an entirely different Crisis, that of civil war, basically. Which, the story had first taken some time to establish, had been about to happen, but... had been called off because of the threat the Mule posed to both sides of the Foundation.
But in spite of both sides of Foundation society working together, the Mule quickly managed to conquer the Foundation. However, prior to that, Kalgan had been visited by Foundationers Toran Darell (an Independent Trader from the planet Haven) and his new wife, Bayta (who came from Terminus, but was part of the underground movement there). On Kalgan, they met a clown called Magnifico Giganticus, who had been running away from his master, the Mule, of whom he was deathly afraid. They had taken him with them back to the Foundation, but now he travelled with them, along with Ebling Mis, to Trantor, to look through the ancient records of Hari Seldon. (The planet had suffered a "Great Sack" by someone called Gilmer around 260 F.E., a date I'm too burnt out on date-reckoning to comb the books for, so I'll take Wikipedia's word for it. At any rate, there are now far fewer people living on Trantor, which has gone, ironically, from being completely covered in metal and ruling the galaxy, to being an agricultural world ruling nothing but itself. Still, the Library survived intact.) They were looking for a clue as to the location of the Second Foundation, which they hoped would be able to stop the Mule. And... I can't really say quite how it ends, except that Mis apparently learned the surprising truth about the location, but didn't get a chance to share it with the others. And that fact meant the Mule wasn't stopped for the moment, but it did lead to his being stopped later, in the next book....
And I guess that's all I can say for now. But while some people might prefer the second book over the first, I think I preferred the first book, actually. Because I found the first three Crises (most of all the second) more interesting than the fourth, and while I found the Mule an interesting character, and loved that part of the plot... I dunno, I simultaneously appreciated the fact that the Plan could be disrupted so completely, and hated that fact. I also was disappointed about not getting to see the intended fifth Crisis actually transpire, as I think it would have been one of the more interesting ones. Of course, the next book would be about putting the Plan back on track, but more of that later. That's actually something I kind of disliked about the next book... no Crises at all. But... it did have its good points, some decent characters and a good story and most importantly, finally introducing us to the elusive Second Foundation (hence the title)....
(On a side note, at one point in the story, Magnifico plays an instrument called a Visi-Sonor, which when I was rereading the book, immediately struck me as the holophonor played by Fry in a couple episodes of Futurama. That was pretty neat.)