In the fourth aliyah Moshe is feeding the flock of Yisro his father-in-law in the furthest reaches of the wilderness (to avoid their feeding on the property of others). Moshe comes to the mountain of G-d and sees a burning bush. He notices that the bush is not being consumed by the fire, so he turns aside to investigate.
G-d calls to Moshe, saying: "Moshe Moshe". (This is the only place in the Torah where a name is doubled without any separating punctuation.) G-d introduces Himself and says that He has seen the afflictions of the Jews. G-d says that he is hereby sending Moshe to take the Jews out of Egypt so that they can be brought to a land flowing with milk and honey. G-d says that he will be with Moshe throughout this matter.