“Isn't it based on a true story?”

You're talking with a friend about a new movie that started playing in theaters last week. Your friend is describing what the movie is about. You think you've heard something about this movie, but you're not sure if it's the same one, so you ask your friend this.

Isn't it based on a true story?

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Isn't (something)(adjective)

Use this when you think something is true, but you want to make sure:

Isn't this, like, your fifth marathon?

Isn't it considered rude to point at someone with your left hand in some countries?

You can also use this when someone says something and you think they might be wrong:

A: Edgar's coming to town this weekend.

B: Isn't it next weekend?

Note that, if the subject of the sentence is plural, you change it to "Aren't..."

Aren't you supposed to be at work right now?

based on (something)

Use "based on" to explain where an idea came from:

Based on some of the research I've read, it's more effective to run more quickly for shorter periods than to run a long time at a slow pace.

I thought, based on the number of customers we were getting, that it was probably time to expand the size of the store.

based on a true story

This describes a book, movie, TV show, or play that's about something that really happened.

This is different from a documentary, which is a movie that shows a real situation as it's happening. It's also different from a nonfiction book, which is a book that presents facts or discussions instead of a story.

Unlike documentaries and nonfiction books, a movie or book that's "based on a true story" is presented in a story format. Some details of the story may be imagined by the writer or changed from how they originally happened. Only the main idea or plot of the story matches something that happened in real life.