A Good Marriage is based on a short story of the same name from Stephen King’s Full Dark, No Stars (book review). Like 1922 (another Stephen King short story) it follows the book plot fairly closely. Some of the details are different, such as the box under the tool bench, Darcy’s fears for herself and her family (which are depicted as hallucinations rather than her thoughts, which makes sense in a movie format), and some of the clues that confirms her worst fears.
It does start a bit slow, but so does the short story, establishing Darcy and Bob’s ‘good marriage’. The average couple, still in love, and two grown kids doing well on their own. Its very difficult to talk about this movie without giving away the pertinent point that the plot revolves around. But I’m going to try to continue.
Skipping to the ending, as the middle of the movie is mostly Darcy coming to terms with the truth, and deciding what, if anything, she wants to do about it. So what does she decide? Darcy’s family is her world, if she tells the truth, her children will suffer, so there’s only one solution left. That solution is to eliminate the problem.