Is the word beginning a noun?

No, "beginning" is not a noun in the sentence "The beginning of the story was exciting." In this sentence, "beginning" acts as a noun.

However, "beginning" can be used as a verb in other contexts:

* "The new year is beginning."

It can also be used as a gerund, which is a verb acting as a noun:

* "The beginning of the story was exciting."

So, the word class of "beginning" depends on its function in the sentence.