God's covenant people, from whom will eventually come the serpent crusher, who will bless all nations, are in Egypt. Like Abram, their father, they entered Egypt due to famine (Genesis 12:10), they are brought out through great plagues (Gen 12:17), and they will plunder the Egyptians (Gen 12:16, 20).
But right now, the situation's not looking good for the Israelites. They are slaves, being treated 'ruthlessly' (Ex 1:14). Although Pharaoh's initial attempt at Eugenics failed (Ex 1:17), the blood of Israelite boys was surely spilled in the Nile, after Pharoah commanded 'all his people' to murder the young Hebrew males (Ex 1:22). They have been crying out for 80 years (2 x 40, 2 periods of testing), and now God answers definitively.
So Moses and Aaron bring about the first plague; the water of the Nile, and then the rest of the land is turned into blood. As we've seen, there was no shortage of blood for the LORD to use, but the sheer quantity is remarkable. Whose blood is it? It seems to be the LORD Jesus creating blood cells ex nihilo (out of nothing). And as the Egyptians take this ex-water in their wooden and stone vessels (v19) they find that it stinks, and they cannot drink. This is a cup of judgement.
Contrast this with Jesus much later, at the Wedding in Cana (John 2). That time, he created grape cells out of nothing, and it tasted like fine wine. That was a cup of blessing. Later still, Jesus' blood would be poured out like wine, and to those who received it in faith would receive blessing, and those who profaned it drunk judgement on themselves (1 Cor 11:29).
Back with Moses, and despite seven days of foul stench across Egypt, Pharaoh's heart remains hard (Ex 1:22). A week passes, and the day of the LORD comes again (you can call it the LORD's day if you like). Pharaoh has failed to respond to the word he received last week, and it's time for another cup of judgement.
This time, the curse comes up from the water in the form of frogs. Frogs are 'creeping things' if you use Biblical categories. Like the serpent who was made to crawl on his belly (Gen 3:14) the frog is unclean. My wife has a minor obsession with frogs, thinking that they are green and cute, which is why we have a frog-shaped CD rack. But frogs in the Bible are always associated with judgement.
That's certainly how Egypt took them. Although Pharaoh was impressed when his magicians replicated the water into blood routine, when his magicians filled the land with even more frogs (Ex 8:7), Pharaoh starts to repent (v8). He sets a deal up with Moses. Make the frogs go, and I will let your people go. But when the LORD took the plague away, and everything settled down, Pharaoh once again, hardened his heart, changed his mind, and kept the Israelites as slaves, just 'as the LORD had said (v15).
All that the LORD has said through his prophet Moses, is coming true. His mighty hand is being stretched out across Egypt to rescue his people. Although Pharaoh seems to have outsmarted the LORD, and got away with breaking his promise, there is yet more judgement to come.
Pharaohs, MPs, vicars, husbands, parents make promises to us; very often that under their leadership, we will receive great blessing. But the LORD is the only one who can promise blessing, in the full knowledge that He will surely do it (1 Thessalonians 5v24).