The principal aim of biblical theology is to engage the hermaneutical challenge: to see where each passage and book fits into the whole. In a classic take strongly inspired by (and referencing) Goldsworthy, Roberts picks out the main events in the storyline of the bible, from creation, fall, exodus, exile, Jesus, and second coming. He assembles these into a picture of a coherent narrative, that of God's final plan for the new heavens and new earth being gradually revealed to his chosen people over thousands of years.
The tone, as in all of his books, is remarkably simple and direct, with plentiful anecdotes and engaging explanation of passages quoted. This makes it ideal for someone new to the ideas presented, such as a young Christian or anyone struggling to identify and understand the flow of the bible, which is after all a rather massive and daunting book. There is significant cross-over in theme with "Turning Points", but "God's Big Picture" is instead aimed more at Christians, while "Turning Points" is a bible overview I think intended to be more evangelistic.