The keyword phrase "patriots day real story" functions grammatically as a noun phrase. The main point, or head of the phrase, which determines its core part of speech, is the noun "story".
In this construction, "story" is the central element being described. The other words act as modifiers. "Real" is an adjective that specifies the nature of the story (factual, not fictional). The term "Patriots Day," itself a proper noun, functions here as a noun adjunct (or adjectival noun), indicating the subject matter of the story. The entire phrase refers to a specific type of narrative: a factual account concerning Patriots' Day.
Identifying "story" as the main noun is crucial for article development. It dictates that the article's primary focus must be a narrative or a factual recounting of events. The modifiers "real" and "Patriots Day" then define the scope of that narrative, requiring the content to be an authentic and accurate account of the historical events commemorated by the holiday or the 2013 Boston Marathon bombing, which is the subject of the film Patriots Day.
