Apparently, this is the second volume of Luke's book, although it comes after John. At least, it's addressed to Theophilus, as was Luke. It's about what happens in the immediate aftermath of Jesus' death.
His first decree is to stay in Jerusalem until they get a gift from god, which will be the holy ghost. They ask if Jesus is going to restore Israel to the Israelites, and Jesus is vague, saying this isn't his jurisdiction, but leaving the door open by saying that it's god's call. He does promise that the holy ghost will infuse them with the power to go and endlessly badger people in Samaria, Judea and beyond. Then he goes up to heaven. Well, that was short!
The disciples watch him floating upwards, and when they look around again, there are two dudes dressed in white standing next to them. The dudes promise that Jesus will return in the same way he left.
They trek back to their one (!) room in Jerusalem that all 11 of them share, because even then Jerusalem was really fucking expensive and no one has thought of settling in Palestine yet. Even more people join them, including Jesus' mother Mary, a number of unnamed women (though they're probably also Marys) and Jesus' brothers. There are about 120 of them in total, and I'm not even going to try to figure out how they fit.
At some point, Peter stands up and starts talking about fulfilling a scripture about Judas, which was supposedly said by David while he was possessed by the holy ghost. Supposedly Judas used his reward money for turning Jesus in to the Romans to buy a field, but then one day he fell down in such a way that his guts burst open like a balloon. Never mind that in Matthew he hung himself! Apparently, by writing a whole new gospel, you can fan-wank this bit so that as he was hanging himself, he fell down and that's how his guts burst open. Whatever you need to tell yourself to get through the day, I suppose. They rename the field Aceldama, that is to say, The field
of blood (v. 19), which fulfills something in the book of Psalms. Anyway, what Peter has been building up to with this bit of salacious gossip, is that now they have to replace Judas. But they can't decide between Joseph Justus (a porn name if I've ever heard one) and Matthias, so they pray to god to tell them which one to choose. Now, I know there are Christians out there who make every decision this way, asking god whether to wear the red nail polish or the pink, the strappy sandals or the mules, but it just seems so time-consuming. Like, what if god is busy telling three different Republicans to run for president and you're late for work? Is your boss going to accept 'Sorry, I was waiting for god to get back to me about whether to shave today,' as an excuse?
Oh, no. It turns out they have a very convenient method for figuring out which candidate god favours: they cast lots. And the lots say Matthias.