Why does the Midrash single out the frog when there were nine additional plagues? The second plague was key because both Pharaoh and the frog shared a common appearance of apathy.

The Apathetic Pharaoh

Pharaoh appeared apathetic in his denial of G‑d. One can either accept the existence of G‑d or reject it; but Pharaoh did neither -- he simply ignored it. He regarded himself as the creator of his universe and never bothered to deny the true creator. He treated G‑d as a non-entity, one that does not even merit denial.

The Apathetic Frog

In a sense one can argue that the frog too appears apathetic. Some creatures are clearly beneficial to the ecological system and enhance the lives of all creatures on the planet. Some creatures appear destructive, selfishly treating themselves to their every wish with no regard to the delicate balance of nature. Some creatures, such as the frog, neither contribute nor abuse, they simply are.

A Fitting Match

Both Pharaoh and the frog seem apathetic; Pharaoh to G‑d, the frog to nature's cause. It was only fitting that G‑d combat Pharaoh's apathy with a creature that itself appears apathetic.

Reflection

By using the frog G‑d communicated another important message. The outward appearance of apathy is simply false. The frog's appearance of apathy disappeared as he responded with alacrity to the call of the Almighty. Though some appear outwardly so, deep down we all believe. We all have a mission to perform. We all play a role in G‑d’s overall scheme.