In the fourth aliyah Moshe tries to impress upon Pharaoh Who sent him by Moshe casting down his rod in front of Pharaoh and it turning into a serpent. But Pharaoh isn't impressed.
Next are the first two plagues of Egypt: blood and frogs. Moshe meets Pharaoh early in the morning by the Nile River. (Pharaoh maintained that he was a god and didn't need a bathroom but Pharaoh would sneak off to the river early in the morning to use the bathroom.) Moshe tells Aharon to smite the river. The river and all the water in Egypt (including in vessels) turns to blood and all the fish die. Moshe specifically does this through Aharon because Moshe was protected by the river when he was a baby (says Rashi). The plague of blood lasts seven days.
Next, the plague of frogs begins, where frogs come out of the river and cover the land, enter all the houses, and their ovens and their bowls. Rashi says that the frogs even entered the internal organs of the people and the frogs croaked there.
The Lubavitcher Rebbe explained that a lesson we can derive from the frogs action is that even though the frog hates heat - that's why they are always in or near water - nevertheless, since they were on a mission for G-d they did things against their inherent nature.