Episode 5 of LTNS starts with another awkward sex scene between a couple, and we soon find out that the woman is the one that Samuel calls at the end of the previous chapter. She and Samuel meet outside the hospital and decide to take a walk.
Meanwhile, Woo Jin runs into a mysterious man outside their door, and when he runs away, she sees that he has written a threatening message on her door, “You reap what you sow.” She tries to call Samuel, but he does not pick up, and she decides to visit the hospital. Once he picks up the call, Samuel lies that he is getting ready to sleep, unaware that Woo Jin is sitting on his empty hospital bed as she makes the call.
After talking to her sister about Samuel being caught in his lie, she thinks that Samuel might be cheating and asks her to be on the lookout. The next day, when she picks Samuel up from the hospital, he insists on driving, saying he wants to return to work as soon as possible.
Unbeknownst to Samuel, Woo Jin installs a ring camera and runs into the mysterious man. After Samuel drops her off at her workplace, she sees through the camera that a woman comes to their door, and they leave together. Woo Jin decides to take three days off work and investigate the issue.
Woo Jin starts by searching through their house for a clue, and she sees the woman’s picture with a message at the back indicating that she is Samuel’s source of happiness. Next, she rents a car with enough horsepower to outrace Samuel’s and secretly follows him.
Memories of the past start to flash through her mind, and she remembers the promise they made to each other while dating, saying that they should openly talk to each other when feelings change, and if those words are not spoken out, the other should always be at ease. Samuel promises to tell her everything, whatever it might be.
Woo Jin follows him around for a while throughout the night before the scene suddenly goes back to one year ago when Samuel and the lady first meet at a self-cleaning car wash. She borrows his wheel cleaner, and she helps him clean his car. Later, Samuel finds out they are neighbours, and she starts frequenting the house when Samuel is around.
The visit begins with the return of the wheel cleaner that Samuel lent her, and they go to have a beer together, sharing food. Eventually, Samuel invites her into his house. Her name is Min Soo, and the relationship extends to “Muel,” as she calls him, fixing the drainage in her house to eliminate foul odour.
They continue to get closer, sharing information and tips about cleaning, calling themselves “cleaning mates.” Their spouses start to notice changes in the house regarding cleanliness and appreciation without knowing that the changes arise from a cleaning affair brewing up in the background.
One day, Min Soo’s husband comes home unexpectedly, and Samuel has to hide while Min Soo feeds her husband sleeping pills to give Samuel time to creep out of the house. After that situation, Samuel decides to end their “cleaning relationship.”
Presently, Min Soo calls Samuel and passes by a chemist on his way home. Woo Jin is still hot on his tail, and she finds out that the woman Samuel has been close to is their neighbour. Samuel takes the medicine to Min Soo, and she asks him to step into the house. Although initially reluctant, he eventually steps in, and Woo Jin captures it on her phone.
Woo Jin walks to the door and stares at it, almost crying, but she suddenly turns savage. She buys a can of fuel and pours it over their and neighbour’s doors. She plays around with the match for a while, contemplating setting it on fire, looking extremely frightening. At the end of the episode, she sparks the lighter.
It turns out that the mysterious woman is someone with whom Samuel has an emotional relationship while cleaning. As weird as it sounds, Samuel and Min Soo developed a deep connection to make her the first person he thought about after his accident.
Min Soo seemed equally emotionally invested in their relationship, as seen from their depressed mode after Samuel broke it off. However, it begs the question, can we say he was cheating on Woo Jin?
There are different types of affairs in a relationship, and this episode shows that betrayal is not only about the physical, like the jerks that Samuel and Woo Jin were chasing, but there is also emotional betrayal. What will be Woo Jin’s reaction? Will she torch the houses?