9 timestamped notes explaining the Korean cultural context in this episode — customs, idioms, food, honorifics, and history, at the moment each appears on screen. These notes discuss events across the whole episode. To see them revealed scene-by-scene as you watch, use the episode page to find an airing.
서봉제철소 (Seobbong Steel Mill) is a fictional factory that ties together the cadmium evidence used to convict the imprisoned man. Cadmium is a heavy metal released as a byproduct of certain smelting and refining processes, and prolonged workplace exposure leaves traces that can show up in hair and body tissue — which is why detectives originally zeroed in on factory workers.
Tiny handlebar-mounted dash-cams are genuinely popular among Korean cyclists, partly because hit-and-run incidents involving bikes are common and partly because Korean insurance and traffic law rely heavily on video evidence. The fictional 지오캠 device is a nod to real brands like Mio and Insta360 that market to this community.
In Korea, 파워 블로거 are bloggers with large followings on Naver Blog who receive free products from companies in exchange for honest-sounding reviews. The arrangement is so common it has its own informal economy, and receiving a pre-release gadget to write up — as this character did — is a standard perk. The catch: posting detailed photos of your home address to show off the item can make it easy for thieves to find you.
The character name-drops 유주얼 서스펙트 — the 1995 Hollywood film 'The Usual Suspects,' famous for a twist ending where the most harmless-looking person in the room turns out to be the mastermind. Invoking it here is shorthand for 'I can't believe someone who looks this gentle could be a serial killer,' a disbelief that is central to the episode's theme.
영치금 is money that family or supporters deposit into a prisoner's official account at a Korean correctional facility. Inmates can use it to buy approved goods — clothing, snacks, personal items — from the prison commissary. The detail that the character bought clothes with his 영치금 and hand-sewed fabric patches onto them shows both how he spent his time and how his DNA ended up on the garment later taken by Kang Deok-su.
Korean investigative journalism and crime shows borrowed the English word 'signature' directly — 시그니처 — to describe the ritualistic details a serial killer repeats beyond what is strictly necessary to commit the crime. The crescent-shaped apple slices and the use of stockings as restraints are being presented as the killer's psychological calling cards, distinct from mere 'modus operandi.'
The broadcast's argument hinges on the convicted man being 선천적 청각 장애 언어 장애 — congenitally deaf and speech-impaired from birth. If he cannot hear speech or produce spoken words, he could not have responded verbally to the victim's pleas the way the surviving witness describes. This kind of disability-based alibi is rarely raised in Korean wrongful-conviction cases, making the point legally significant within the drama.
Cadmium compounds were historically used as yellow and orange pigments in industrial dye processes, including denim fabric dyeing. Korean environmental regulators have flagged cadmium contamination around textile factories for decades. The show is pointing out that the original investigators treated cadmium exposure as unique to the steel mill, ignoring that several other nearby factories could have produced the same trace evidence in a different worker.
국과수 is the shorthand for 국립과학수사연구원, Korea's National Forensic Service, the sole government body authorized to conduct official forensic analysis — DNA, toxicology, ballistics — for courts and police nationwide. Evidence submitted to 국과수 carries legal weight that private lab results do not, which is why characters treat getting a sample tested there as a decisive step.