The book opens with our narrator, John. He's just a mild-mannered Christian guy living in exile when—pow!—God hits him with a doozy of a revelation (hey, that's not a bad idea for a title). John is told to write down everything God shows him, and boy is God going to show him some strange things.
First, John sees a vision of Heaven. There he meets God and Jesus and all sorts of other angels, elders, and weird six-winged creatures that inhabit the heavenly realms. After a brief how-do-you-do, John sees Jesus come forward (in the form of a seven-eyed lamb) and open seven seals. Each of the seals lets loose God's fury on the Earth. We're talking rivers of blood, plagues of locusts, giant hailstones, twenty more seasons of Jersey Shore. The last seal unleashes seven angelic trumpeters, which in turn unleashes seven bowls of God's judgment. Yep, God really has a thing for sevens… and pouring down his wrath on non-believers.
Rivers of blood not freaking you out enough? While all this is happening, John sees some pretty hideous creatures lurking around. A giant red dragon, a beast with seven heads, and another beast who also happens to love the number 666. John is told that these baddies have been busy convincing the people of Earth to worship them instead of God. They also like to kill Christians and just generally be evil. Things are not going to go well for them.
After God sends all the plagues to Earth and loads of people die (eek!), he has the armies of Heaven wipe the dragon and the two beasts off the face of the Earth. Literally. They get thrown in the Lake of Fire… along with all their followers who refuse to worship God.
As for all the faithful Christians who stayed strong and kept faith in Jesus, they get to go to a splendid city in the sky where the streets are paved with gold and diamonds and nothing bad ever happens to anyone ever again. It's all sunshine and lollipops and the eternal light of God.
John ends the whole book by assuring us that everything he saw is totally and 100% true and that it's all going to happen for real very, very soon. The hour is at hand, so repent, y'all. Don't say that a seven-eyed lamb didn't warn you.