WARNING: The following contains spoilers for Mars Red Episode 4, "Unknown Song," now streaming on Funimation.
Mars Red's Episode 4, "Unknown Song," sees a mysterious vampire epidemic spreading through Yoshiwara's Red Light District, leaving behind not only human victims, but vampires as well. While the main squad of Code Zero walks the streets trying to identify the source of the spread, journalist Aoi Shirase takes on an assignment that puts her on a collision course with the one responsible for the infection: a haughty English businessman named Rufus Glenn.
After someone from the government kills her coverage of the mysterious Yoshiwara deaths, Aoi's editor redirects her to something more to their readers' taste: a new line of British perfumes that will be shown at the inauguration of the brand new Frank Lloyd Wright Annex to the Imperial Hotel. Aoi had been looking forward to scoping out the new building, so she heads to it right after work, arriving at the same time as a carriage with a blonde foreigner and a government agent separate; the foreigner heads to the hotel, and the government agent, who the viewers will recognize as General Nakajima's supposed ally, goes out.
Aoi's superpower is striking up conversations with incognito vampires that, unless they are Deffrott, irritate them to no end, and this scene is no exception. The foreign businessman notices her stalling close to the hotel's door and throws her first a penny and then a vial of "perfume" that looks too much like blood to be a coincidence, doing anything within the rules of polite society to make her leave. Uncharacteristically for Aoi, who is usually a ray of sunshine dripping with optimism, she reciprocates his hostile feelings even after finding out that the rude stranger was Rufus Glenn, the parfumier that she's supposed to cover in an article.
When her daytime investigation of the Yoshiware murders yields only rain, she goes to vent to the only person who will listen to her: Deffrott, the ancient child vampire turned actor who lives and works in the Imperial Theater. He's sympathetic to her plight until she shows him the vial of perfume -- a scent that she detested -- and the alarm bells ring inside his head. Aoi, who doesn't quite believe that vampires exist and would never suspect her friend of being one, gifts him the "perfume" -- which is actually a vial of corrupted blood that contains the V-virus. To vampires, the scent is not repulsive, but "cloyingly sweet."
Meanwhile, Code Zero stumbles upon a huge barrel filled with the same vials. After a cursory analysis of the wood, Takeuchi determines that it had been soaked in Scotch whisky, pointing out that whoever had been turning humans into zombie vampires probably has some kind of connection to Great Britain.
The very last scene shows Deffrott and Rufus Glenn talking, with Deffrott, worried that whatever Glenn and his masters are doing will turn Tokyo into a hunting ground, something he never wanted. The conversation is very vague -- a hunting ground for whom? For vampires to hunt humans, or for humans to hunt vampires out of fear of another zombie attack, like in Yoshiwara? While Deffrott ponders, Rufus is more concerned with his hunger for Deffrott's blood.
The events of Episode 4 imply that Yoshiwara is a test site for whatever substance is in the "perfume," which could possibly be connected to the bootleg artificial blood that was flooding the black market at the beginning of the series. But if Rufus Glenn and his British masters are working with the Japanese government, does that mean that there's a widespread conspiracy to fan hatred against vampires and discredit efforts to integrate them into the military? Or is there something entirely different afoot?