Code Geass: Mukei no Arthur Case File - Beginning

Revolutionary calendar year 98, imperial calendar year 1887, summer. Two young men met in a corner of London. One is Conan Watson, a college student who dreams of becoming a doctor and has just left the countryside for the first time in several years. And the other is Arthur Holmes, an eccentric young man who calls himself an inventor and is driven by his own curiosity. The two of them ended up living together because of a mischief of fate or something else that happened. After a certain incident unfolds in London, he gets caught up in a number of mysterious incidents. Conan witnesses the inferences drawn from Arthur's genius observation eye.

(Source: Kadokawa, translated)