This is a summary based on my understanding of the novel. Please also note that this summary contains spoilers. You may want to read the entire book first before proceeding
It all started when Miriam discovered the dead body of Daniel inside the boat he’s renting. Miriam is a neighbor, she lives on another boat.
The main suspect was Laura Killbride. She spent the night at Daniel’s boat, slept with him then argued with him. Laura got angry because Daniel insulted her – so she bit his neck, drew some blood. Got some of his blood on her shirt as she went home. She also injured herself while trying to get inside her house because she lost her key – she unknowingly left it at Daniel’s boat. Miriam took the key when she found Daniel’s body because she wanted to protect Laura. Miriam thinks Laura is as damaged as her.
Laura is a bit unstable – she has a limp, she has a violence history, and this all started when she got hit by a car while riding a bike as a kid. The driver of the car was her mother’s lover. Her mother ended up marrying that man.
Miriam is also unstable – she was abducted as a kid. She and her friend hitchhiked, was taken to a farm by a man called Jeremy. Her friend was murdered, she was able to escape. It was assumed that Jeremy took his own life after the incident.
Another suspect of Daniel’s death was Carla. She is Daniel’s aunt. Daniel’s mother, Angela, died just months before. Their relationship hasn’t been perfect. Carla used to have a son, named Ben. She left Ben with Angela as she went to work, then she just received a call about his death. Ben fell from the study, since the doors were left open. Despite this, Carla tried to forgive Angela. She still visits her sister and still communicates with Daniel. However, based on Daniel’s drawing, it looks like he has a weird fascination of his own Aunt. Worse, Carla saw a book drawn by Daniel. It showed how as a kid, Daniel purposely opened the doors of the study, threw a toy so that Ben would crawl towards the edge. It turns out afterall, Daniel killed Ben and let his own mother take the blame.
An arrest has been made – they arrested Laura because aside from her record and for being the last one seeing Daniel alive, the murder weapon (knife) and a scarf with Daniel’s blood was found in her flat. She didn’t know how it got there. She didn’t know it was the murder weapon – she simply stole Carla’s bag because she needs the money.
When the detectives checked for fingerprints, it turns out the knife and scarf were owned by Theo. Theo is Carla’s ex husband. He is an author whom Miriam claims stole her memoir (he wrote a book about what happened to Miriam). When the detectives asked Theo, he confessed to Daniel’s murder. He said Angela told him that Carla and Daniel was sexually involved, so he killed Daniel. However, he only has one fingerprint on the knife – he was confessing for a crime he didn’t commit because he knows Carla did it.
A neighbor named Irene showed Theo the book that Daniel has drawn. It showed how Ben died. Theo knows that Carla saw that book, he knows that Carla killed Daniel.
Carla finally confessed to Irene that she did murder Daniel. She went to the boat, she just wanted to hear him deny what he drew. However when she saw Daniel, she didn’t even ask and she stabbed him instantly. Carla was sent to jail. Laura and Theo were released.
While in jail, Carla read an article about another death inside a boat. A man named Henry died, he was stabbed as well. Nobody knows who killed him – but Henry changed his name from Jeremy. He was the man who abducted Miriam and her friend. Both Miriam and Theo figured this out after receiving the letters they got from Jeremy / Henry.
The End of Slow Fire Burning by Paula Hawkins.