The Marriage of Heaven and Hell starts off with section called "The Argument." No it doesn't. Yes it does—oh sorry, we got carried away by the title. At any rate, in this section we're introduced to a character named Rintrah, and the dude is not happy. He's raging out, while a "just man" keeps walking along the "Vale of Death."
We also have a "villain," who is leaving an easy path to walk a more dangerous road. In doing so, he's also pushing our just man toward a desolate area, where the just fella starts to rage out, too. The section concludes with an announcement: a new Heaven is beginning, kind of like a grand opening. Then it sets forth a few points that seem worth keeping in mind: a) there is no progress without contradiction and b) being good means passively obeying, while being evil means being active with energy.
The next section is "The Voice of the Devil," which we imagine is a cross between James Earl Jones and Bobcat Goldthwait. This section gives us a list of "errors," which the speaker blames on the Bible and other "sacred codes." Essentially, the gist is that the notion that the body is separate from the soul is a bunch of bunk. The body and the soul are actually one (according the speaker, at least).
Then the speaker praises John Milton's Paradise Lost for getting this idea right, as well as for illustrating how resisting temptation actually makes a person weak. In our speaker's view, the real hero of Milton's book is—wait for it—Satan. The Devil followed his desire, which to our speaker makes him the Messiah. The speaker gives props to Milton for his portrayal.
Our next section is entitled "A Memorable Fancy." It's basically a travelogue of our speaker's trip to Hell, which leads to the next section "Proverbs of Hell." Here the speaker lists a whole slew of short sayings that he picked up in Hell, like "Sooner murder an infant in its cradle than nurse unacted desires" (4.67)—okaaay. In general, these saying are pro-desire, pro-temptation, pro-creativity, and anti-repression.
After this, four sections follow—each with the now-familiar title of "A Memorable Fancy." In the first, the speaker has dinner with the Biblical prophets Isaiah and Ezekiel. He interviews them about how they knew that God was speaking to them. Then they tell him about the origin of religion: poetry. In the next section, the speaker visits a "printing-house in Hell" (5.1) and basically describes the inner workings of the equivalent of an Amazon warehouse in Hell.
The speaker then goes on to mention Giants who formed the earth. He makes a distinction between a group of people called the Prolific and another group called the Devouring. The Prolific are the creative folks who gain energy from following temptation, which the Devouring are tame, obedient, and oppressed. These groups should be enemies with one another, says our speaker.
In the third "A Memorable Fancy" section, the speaker is visited by an angel, who says that the speaker is cruising for damnation. For some reason, instead of being terrified, the speaker wants to know what that might look like. So the angel shows him a flaming abyss where giant spiders will chase him for eternity. Once the angel takes off, though, the scene changes to a pleasant riverbank, complete with a harper.
The speaker then offers to return the favor for the angel. They open up a Bible, which turns into a deep pit, and they both dive in. They wind up in a brick house, within which a bunch of monkeys are chained up and doing unspeakable things to one another (e.g. rape and cannibalism). The angel is annoyed by this, just as the speaker was annoyed by his vision of the spider abyss. The speaker then goes on to diss both Aristotle and Swedenborg—the latter of whom "has not written one new truth" (7.16).
In the final "A Memorable Fancy" section, the speaker sees a devil talking to an angel. He tells the angel that to worship God is to love "the greatest men best" (8.1). He also says that you have to break the ten commandments to be truly virtuous, and that Jesus Christ did exactly that. Initially peeved by the devil's arguments, the angel eventually comes around to his way of thinking and—presto change-o—he turns into a devil. We also learn that this angel was Elijah.
"A Song of Liberty" is our next section, which gives us twenty lines on the politics of the day, including the situations in France and America. The upshot of this section has our speaker calling out for an end to empire and freedom for oppressed people. The final section, "Chorus," calls one last time for an end to religious repression, wrapping everything up with a famous last line: "For everything that lives is holy" (10.2).