Acts opens where Luke's Gospel ends, with Jesus' ascension, and then narrates the birth and spread of the church across roughly three decades, from a small group praying in an upper room in Jerusalem to Paul preaching, under house arrest but unhindered, in Rome itself. Its hinge event is Pentecost, in chapter 2, when the Holy Spirit descends on the gathered disciples amid wind and fire and Peter preaches the church's first recorded sermon; the book treats this as the moment the church becomes capable of its mission. The first half of Acts centers on Peter and the Jerusalem church, including the stoning of Stephen, the first Christian martyrdom, and the conversion of the Ethiopian official, an early sign of the gospel crossing ethnic boundaries.
The turning point is Paul's conversion on the road to Damascus, recounted three times in the book, after which the narrative follows Paul almost exclusively through three missionary journeys across Asia Minor and Greece, his arrest in Jerusalem, and his lengthy legal appeals up to Caesar that carry him to Rome. The Jerusalem Council in chapter 15, which decided that Gentile converts need not follow the full Jewish law, is the book's central theological turning point, resolving the question that most of Paul's own letters argue about from the inside.