He can't commit, so, whenever a relationship starts to get serious, Charlie finds an excuse to dump his latest girlfriend. Hey, if she smells like beef vegetable soup, that's a good enough reason. This time, however, Charlie is really smitten. P.D. James need not worry. Mysteries are supposed to be complex, intriguing, and beguiling, with the audience unsure of the truth until the very end.
Any movie can rise above flaws if it's funny enough.
Humor is often the best paint to hide the cracks in the story. John Cleese's A Fish Called Wanda is an excellent example. Not only is the humor banal, it's only fitfully amusing. For every joke that works, there are half a dozen that fall flat. Apparently, Mike Myers thinks he's a lot funnier than he actually is.