A strict reading of [[WP:OR]] would dictate that whileeven though a source isn't needed to relay events of the plot, even the most basic facets of ''interpretation''—motivations, themes, depth—require reliable sourcing to back up the claims. It's easy to say, for example, that [[Josh Lyman]] blows up at President [[Josiah Bartlet]] while in the Oval Office in ''[[The West Wing]]''{{`s}} second-season episode "[[Noël (The West Wing)|Noël]]"; to say that he did so as part of the episode's portrayal of his breakdown due to yet-undiagnosed [[post-traumatic stress disorder]] without sourcing, that's a murkier call. Using reliable sourcing gives you access to a richer interpretation of the characters and setting that can bolster your ability to give a nuanced summary of the plot.
==For non-fiction, too!==
This problem isn't limited to works of fiction; political books, documentaries, scholarly articles, and history books all have lots of content that might need to be summarized if the work qualifies for a Wikipedia article. However, for political books especially, the main idea should not be to summarize every point and argument made, or the ones that stood out to you.