The first episode traces the development of the four rocky worlds closest to the Sun: Mercury, Venus, Earth and Mars. Born together, they battled the unbelievable violence of the early solar system to become stable planets. For a while each had a moment of hope when they enjoyed almost earth-like conditions. Yet today Mercury is a scorched barren world. Venus is a runaway greenhouse world with a scorching atmosphere and Mars is a frozen desert. Only on Earth do oceans – and life – persist.