The first plague was that the waters of Egypt were turned to blood. Moses and Aaron struck the Nile as commanded, and fish died and the river stank so that Egyptians could not drink the water. The text also extends the effect to other water containers and sources (Exodus 7:19-21).
This unit sits early in the plague cycle, following the initial court confrontations in Exodus 7.
This passage matters because it ties the detail to the larger movement of the book, keeping the answer connected to context rather than isolated wording.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Exodus 7:19-21, then read the surrounding section so the answer stays connected to the passage's flow.
19Yahweh said to Moses, "Tell Aaron, `Take your rod, and stretch out your hand over the waters of Egypt, over their rivers, over their streams, and over their pools, and over all their ponds of water, that they may become blood; and there shall be blood throughout all the land of Egypt, both in vessels of wood and in vessels of stone.`"
20Moses and Aaron did so, as Yahweh commanded; and he lifted up the rod, and struck the waters that were in the river, in the sight of Pharaoh, and in the sight of his servants; and all the waters that were in the river were turned to blood.
21The fish that were in the river died; and the river became foul, and the Egyptians couldn`t drink water from the river; and the blood was throughout all the land of Egypt.