Dept. of Speculation begins with an unnamed female protagonist questioning the nature of relationships. She thinks about some of the people that she’s met, and how she used to focus on her work. In the beginning she was a writer and a fact checker, first working for a scientific magazine. She thought if she focused solely on her career she wouldn’t feel lonely in life, but that wasn’t the case. Her friend whom she refers to as ‘The Philosopher’ introduces her to a man who creates something called soundscapes.

The man ends up calling her and the two are soon dating and getting to know each other. That year, the two end up traveling together and seeing more of the world. The couple eventually ends up moving in together, in a small apartment that has a problem with bugs and mice. They both seem to have their doubts about the relationship, as if they have rushed into things too quickly, but the protagonist hints that they dismiss these because neither wants to be alone.

The protagonist ends up getting pregnant with her lover’s child. She adores the baby when it’s born. Their child is very loud which is something that the protagonist initially worries about because it makes it hard to take the child out or have people over. Her husband is forced to take a new job. Instead of making his own soundtracks, he starts doing sound work for commercials. It isn’t the job that he wanted to move to, but the pay is better and the family had been struggling financially. The protagonist ultimately ends up having to take on different work as well. She starts to ghostwrite for a client she calls ‘Almost Astronaut’, who is fascinated with space and trying to write a book about it. The protagonist works as a teacher as well.

As time goes on, the protagonist starts to think more about their lives. While she loves her daughter unconditionally, it seems like her life has become emptier ever since she decided to become a mother. The family starts to have greater problems with bugs in their apartment. Despite their best efforts, they are having trouble getting rid of them. After another year, the family is finally able to find a new apartment to move into. During this time though, the protagonist continues to think about all the other things that they could be doing to make their lives more exciting, clearly longing for something more.

The narrative shifts and for the rest of the book the protagonist stops referring to herself as “I” and instead starts to use the third person and calls herself ‘The Wife’. It’s clear that she’s unhappy with where she is with her life. Not only that, but it’s revealed that her husband is unhappy as well and has started having an affair. The two attend therapy, but things are still rough. The wife tries to confront the girl he was cheating with, realizing that her husband picked someone that looked like a younger version of her.

The wife repeatedly contemplates divorce, and despite the best efforts of her husband, it is clear that their relationship is shattered. To make matters worse, he starts to relapse. He claims that he hasn’t been with the girl again, but his wife finds out that he’s been sending messages to her. The wife is constantly thinking about her relationship and the way that she’s been living her life, how much she’s changed since she was younger.

In an attempt to fix their relationship, the family moves out to the country. At first their daughter has trouble with it, but she ends up adjusting to the move. The husband and wife still occasionally fight after the move, but over time she starts to keep quiet, although she has fantasies about a secret life where she runs away. By the end of the book the wife has started referring to herself in the first person again. The book ends rather abruptly with the protagonist sending her child off to school during the winter with the protagonist seemingly feeling defeated in life, but having come to accept her fate.