Surprised and delighted to discover that there's a whole wiki devoted to this game and how it works. Take a look
-- be warned, though, spoilers abound; the game is a lot more fun if you play through at least a few cases and see how things unfold for yourself.
It's a really interesting game. Rather than things unfolding as a single, ongoing campaign, it's instead split into smaller "cases" that take 30-60 minutes to complete. Each case is led by a criminal mastermind, and it's your ultimate aim to capture all of these masterminds. They're not the only participants in each plot, though; they have a team of accomplices helping to make their plan come to fruition, and in order to get the best evaluation on each case you're going to need to think carefully about who does what and arrest people in the right order -- because arresting someone before they fulfil their role in the major crime of the case will break the plot and cause the other participants to go into hiding, preventing you from catching them. This may include the mastermind.
Let me give you an example. A case I played today saw a group attempting to kidnap a scientist who knew important information about satellites. The sequence of events ran something like this:
- Organiser sends a message to Financer
- Financer withdraws money
- Financer holds on to money
- Financer hands off money to Kidnapping Mastermind
- Kidnapping Mastermind sends message to Kidnapper
- Kidnapper kidnaps target
If I were to, say, arrest the Financer before he handed the money off to the Kidnapping Mastermind, the latter party would realise that something was up and bolt, as would the Kidnapper themselves. I'd have prevented the crime, but at the expense of being able to arrest some of the participants. In other words, I'd get credit, but not quite as much as if I'd have arrested everybody involved in the plot.
What you can do in some situations, then, is seek incriminating evidence while you're infiltrating an agent's headquarters. This allows you to "turn" them to act as a double agent, which means they will go back to performing their role as normal, but feed you information as they go, allowing you a much better chance of being able to capture everyone involved quickly and efficiently.
The minigames involved in cracking the cases are simplistic but fun, and the whole thing hangs together a whole lot better than Meier himself reportedly believed it did when he first released it onto an unsuspecting world.