In Act I, also known as the big setup, we are introduced to Michael Valentine Smith, and we learn about his origins on Mars. We come to understand the political struggle taking place around him, but we also begin to see how completely alien his understanding of our world and society is. The act ends when Jubal brings resolution to Mike's political entrapment, and Mike can begin grokking, and joining, humanity.
Act II begins with Mike's visit to the Fosterite church. His experiences there lead him toward self-reliance and his quest to grok human nature. First stop: be a carnie. He works a bunch of random jobs to discover how people and society function. In the meantime, he studies all the world religions, only to find that bits and pieces of every religion work but none of them contain the complete truth. His big revelation comes while watching monkeys beat each other up at the zoo—naturally. Mike suddenly realizes that he groks humanity by understanding humor, and he laughs for the first time. After a good chuckle, Mike decides to start his own religion. Bottom line: he wants to help humanity.
Act III almost starts with Ben's story about how the Church of All Worlds operates, but in reality, this part is more background information for the true meat of Act III. The good stuff kicks off with Mike's church burning down. Jubal and Mike discuss everything that Mike has tried to accomplish with his church. Mike thinks he's a failure, but Jubal points out his successes, bringing resolution to Mike's internal conflict. But when our favorite Martian confronts a mob, he is killed—just like that. The rest of Act III ties up the loose ends, telling us what happened to each character, including Afterlife Mike.