From Dictionary.com, conspiracy means "a combination of persons for a secret, unlawful, or evil purpose."

What's key in this definition is "combination"; the word "group" is used in other definitions, such as "a secret plan by a group to do something unlawful or harmful."

In spy noirs, there are many such combinations, which reoccur in specific and different kinds of spy noir plots.

1. spy rings, which are certainly conspiratorial (and which may be busted by "good spies").

2, resistance fighters, whose underground activities are certainly conspiratorial. Are they secretive? Of course. Are they unlawful? Sure, because they are waging campaigns against the current authorities in their homelands (and if discovered, they will be executed).

3. fifth columnists, whose secretive plots are certainly conspiratorial.

Thus, a large number of spy noirs involve conspiracies.