Although not the best of the Charlie Chan series, this is a cut above the later Sidney Toler Chan films. However, what really makes it worth seeing is that the story takes place during the 1938 Munich Crisis (September 1938), yet was produced before WW-II began (September 1939). The setting is Paris, which is blacked out due to the threat of war, and while the French armed forces are busy mobilizing. The city-wide blackout explains the title, as "The City of Light" had been transformed into a "City in Darkness".
There are plenty of lame gags involving the distribution of gas masks, and people panicking due to false air raid alarms. Within a few months of this film's production none of those things would be laughing matters anymore. In fact, although produced prior to the outbreak of WW-II, the movie was not actually released until December 1939, by which time the war had actually begun.
In a sense, therefore, "City in Darkness" represents a significant moment in history that, one might say, has been preserved in a drop of amber. It was the moment when one world crisis was averted, leading to the preservation of world peace for a last few happy months before the final unleashing of Armageddon. For that alone, if for no other reason, "City in Darkness" is still worth a look.