At the end of the previous aliyah Pharaoh says he will let the people go if the frogs go away. So in this, the fifth aliyah, Moshe asks G-d to make the frogs go away on the day Pharaoh asks, so as to impress Pharaoh with the power of G-d. The frogs die and Egypt smells very bad from their odor. But Pharaoh backs out, and doesn't let the Jews go. Then two more plagues happen in this aliyah:
G-d tells Moshe to tell Aharon to smite the dust of the earth so that it becomes lice (Moshe couldn't smite the dust himself since the dust had hid the Egyptian Moshe killed - Rashi). The plague of lice begins, with lice covering man and beast -- Even the magicians of Egypt were impressed and they told Pharaoh that this is the "finger of G-d".
Pharaoh doesn't change his mind, so G-d tells Moshe to warn Pharaoh about the next plague, which will be the plague of noxious animals. Serpents and scorpions will cover Egypt and fill their houses, but they will leave alone the land of Goshen, where the Jews live.
