Joker Game is definitely a mature anime, not in a sexual way, but information wise. It is 90% talk, 10% action.
The plot is about Japanese spies, how they are trained, why they exist, etc. One of the men, Sakuma believes spies are cowards and looks down on them as being cowards. They reciprocate by outsmarting him and making him look dumb in front of the public and private superiors. One spy even sets him up in a situation where he has to commit suicide. It is very manipulative. I am not sure if this is based on confirmed history, rumored history or was created.
A lot of the animation has a heavy tone, with dark colors and heavy sharp shadows. The Head of the organization eyes are often hidden out of frame, or we see him from the back. When we do see him, he is skinny and gaudy, accentuating all of the sharp dark shadows.
Animation wise, it is solid, clean lines, no exaggeration or abnormality in the characters movements. This is a more ‘based on reality’ type anime. There is little to no humor or comedy in the episode because of the serious nature. The characters do smoke cigarettes a lot and there is a constantly moving swirl of smoke at the top of the room.
Personally I found the episode boring. The episode was an explanation of the spies and of Sakuma who hates spys. Perhaps later the spies will be more proactive instead of shadowy, but I don’t have high hopes for that. But that is the direction the author and production team wanted to take this anime this definitely has an audience, it just is not y slice of pie.