Here is a screenshot from Googling it.
The main idea is the sum of the plot, basically. So you should ask, "What is the most important thing happening?"
In "The Incredibles", the Main Idea is that family takes care of family.
In "Unwind", the Main Idea is that all lives have value.
In most children's shows, the Main Idea is teamwork or friendship makes people stronger.
Answer:
That was a more incredible film //////// or that was a more incredible film than the movie we watched yesterday.
Explanation: