It is read ものがたり (monogatari).
This word means story or tale. It is a noun, so it can be modified by この, giving この物語 = this story.
は is the topic marker. It marks この物語 as the topic of the sentence.
A natural way to think about it is:
English does not usually mark topics this way, so this can feel unfamiliar at first. In Japanese, は often introduces what the sentence is going to say something about.
With い-adjectives like 面白い, Japanese does not need a separate word for is in the plain form.
So:
The adjective 面白い can directly end the sentence. Japanese often works this way, so English learners need to get used to sentences that feel like they are missing is, even though they are completely correct.
面白い is an い-adjective.
You can tell because it ends in い, and it behaves like an adjective that can directly describe something or end a sentence.
Examples:
Because it is an い-adjective, it does not need だ in this sentence.
This is the plain form. It is not rude, but it is less formal than polite speech.
A polite version would be:
Both are correct:
Yes, but the nuance changes.
この物語は面白い。
= As for this story, it’s interesting.
This presents this story as the topic.
この物語が面白い。
= This story is the interesting one or It is this story that is interesting.
This puts more focus on this story itself.
For a beginner, は is the natural default here if you are simply making a statement about the story.