Jeremiah started his disastrous (meaning, under a bad star) career as a self-proclaimed prophet during the reign of king Josiah, which puts him 3 kings later than his predecessor Isaiah. Hearing voices in his head, he naturally assumed they were god's; to boost his credentials, he claims that god already chose him as a prophet before he was born and exalts him above all others. (A neat trick that most subsequent prophets will copy - after all, who could ever verify such a claim?, culminating in Mohammed claiming to be the biggest, baddest and last of them all, so any prophet popping up after him is a fake. Poor Joseph Smith never really had a chance.)
As he is severely disappointed in the Israelis, god sends Jeremiah to Israel to try & win back believers. The funny thing is, that although god likens Israel to a whore (along with her "treacherous sister Judah"), he seems really keen, worried even, to win back the favors of his flock. (Understandably so: what is a god without believers? Just another fairytale... after all, today's religions are tomorrow's myths. Apparently, it's simply not possible for god to acquire new believers on his own - he needs his "chosen" people, and specifically, mediators like Moses or Jeremiah. His usual shtick of fire & destruction tends to turn people's noses in the right direction as well, but it's only ever temporary: as soon as he stops killing and destroying, people forget about him. Having believers stick to their beliefs is a never-ending job.) As a sign of their goodwill, Jeremiah asks the Israelis to circumcise before the lord, and even to "take away the foreskins of your heart" (4:4). He really has this thing against foreskins, beats me why. They seem to be some sort of symbol for evil (why god ever bothered to create them remains a mystery shrouded in an enigma), as can be seen from the fact that even unbelievers' ears are described as "uncircumcised".
God tries to be nice for a bit, pleading for the love and devotion of his erstwhile followers, but soon grows tired of this cloying sweetness and brings out the unquenchable fire and utter destruction. That's more in character. Swiftly finding his usual pace, he follows up with a detailed description of wholesale destruction and desolation. (It's still funny to see how a purported god of love can only beat followers into submission by threatening to destroy their whole world... riddle me this, o christians.) As with Isaiah, this newly bipolar version of god seems to constantly swerve between tearful bewilderment (boo-hoo, why are all those lovely Israelis so mea-hean...) and barely contained rage (rivers of blood, desolate lands, mountains of dead people). No wonder everybody keeps well away from him: this god is emotionally about as stable as a melting nuclear reactor.
A certain Pashur, a priest's son, hears Jeremiah's, well, jeremiads, and has him thrown in jail. But with a little help from god, Jeremiah frees himself & happily continues doom-preaching. For good measure, he also includes a gratuitous prophecy about some future king of the line of David who will be super pious & awesome. (David, as we saw, was the progenitor of half the population so it would be hard to find a new ruler who's not somehow related to him, so as a prophecy it's rock solid.) For emphasis, he sometimes indulges in rather cheesy similes, like a damaged girdle, a bad piece of pottery or two baskets of figs (one good, one "evil"). Pathetic. He preaches for 10 years, but, unsurprisingly, no one listens nor cares. At some point though, the people are really fed up and unanimously decide he'll have to die for his continued wailings. But he quickly gets out of his predicament, stating once more he's only delivering god's word. As they don't want to risk the wrath of god, they let him go. (In those superstitious times, you really could get away with that... and it's even good for getting rid of competitors. For instance, a certain Hananiah, who countermands Jeremiah's instructions, dies some months later. And let's just call it a coincidence that he was a really old guy.)
At some point though, Jeremiah is once more imprisoned. God keeps informing him about the political status quo, very practical in those pre-internet times. Going back in time (a trip to memory lane while languishing in prison?), he then recounts the days when Jehoakim son of Josiah ruled (another bad king, as we saw). As he is placed under house arrest, he dictates his prophecies to a friend, Baruch. (Probably his only friend... he can't have been very popular.) The officials, after hearing Baruch read from the scrolls and inquiring whence he got his information, tell him to hide together with Jeremiah as they fear the king will execute them. (In absentia, he contents himself with just burning the scrolls; unperturbed, Jeremiah keeps on dictating new prophecies to Baruch.) Back in the present, the current king, Zedekiah, is slightly apprehensive about incarcerating a prophet, so he orders the guards to keep him in the prison's courtyard and feed him well. But the guy just keeps on lamenting, so eventually he is holed up again, this time even sunk into the mire of a dungeon. But others, upon hearing this, hoist him out and return him to the prison's courtyard where he spends several more years.
God (through his puppet) warns the people of the upcoming Babylonian invasion but comforts the people by telling them he will visit them in their captivity and free them a mere 70 years later. (How considerate.) And indeed, at some point Israel and Judah are invaded by the Babylonians (told you!); their king Nebuchadnezzar frees our hapless hero. (He might be a conquering bastard but he's no fool... don't fuck with YHWH, even if he's not your god and technically you don't even believe in him. A bit like Pascal's wager, really.)
A certain Ishmael slaughters a Babylonian governor (plus, for good measure, many others). Guards try to capture him but he escapes. So the officials turn to Jeremiah and ask him if he'd like to ask god what to do; they will do his bidding. After 10 days (bad connection?) god answers, but predictably his answer doesn't really help so the people just ignore it. God throws another tantrum - more promises of death & destruction (temper temper!), with, by the way, another mention of dragons (who will be the only survivors, living amongst the ruins of cities). He then continues his rant against the Babylonians and the Chaldeans (who conquered all Israeli cities). Read in succession, it once more highlights the endless futility of this terribly tedious book. God punishes his own people - god gets contrite, saves his people & punishes the oppressors - his people forget about him - more punishment - etc., an endless cycle of misery and woe, completely fucking pointless. The book doesn't even have a proper ending - the last chapter is just a recap of the Babylonian siege & oppression, as seen before.
As a bonus, we get an additional book of his writings, The Lamentations Of Jeremiah. As we saw in XXVIII, these were apparently written after the death of his patron, king Josiah, although the king isn't even mentioned; it's just four more pages of bleatings about the degenerative state of the world and how god will punish & destroy everything because he loves his people so much. One interesting detail: "he giveth his cheek to him that smiteth him" (3:30) - Jesus, who knew his literature, wasn't being original, just rehashing old sayings. All in all, another book we can easily do without. And there's 14 more prophets to come. O brother.