2400 A.D. is first and foremost an exploration and puzzle game. The exploration is done in real-time -- and the timer is ticking! As a newcomer to Metropolis, every 2000 ticks you must check in at the local Tracking Office, which resets the timer and lands you a social demerit (SD) if you are late. Receive five SDs, and you'll be put in jail. At first glance this kind of time restriction seems to be the game's most annoying feature, even if it certainly adds to the atmosphere. The good thing about it, however, is that, 2400 A.D. being an RPG, the timer is also implemented as a role-playing feature. You do get penalized for violating the time restriction, but you can still choose to ignore it -- and suffer the consequences, with robots attacking you on sight! Or not suffer, provided your character is strong enough to cope with them. The game is trolling you, and you can troll it in return. After all, even getting sent to prison is no big deal, for reasons that you're better off discovering for yourself.
Once you get accustomed to the timer, you can start gradually exploring the entire city of Metropolis. The city is quite huge and sandbox-ey, featuring several underground and above-ground levels and divided into five large zones: the downtown, the administration complex, the university campus, the Megatech industrial zone, and the apartment district. Each zone except the downtown requires a corresponding Zone Access Card to roam it freely -- alternatively, you can choose to enter a zone illegally and, again, face the consequences if caught -- and each is populated by a large number of NPCs, plot-critical and not. The conversation system is keyword-based, and NPCs aren't exactly the wordy types, guarding their secrets carefully and frequently waiting for you to type in a specific keyword that you can only find elsewhere. (Even in my two playthroughs, I haven't been able to get all possible information out of NPCs, and I tend to explore everything very thoroughly.) NPCs are the only source of your in-game knowledge and talking to them is the sole way to advance the main quest. Accordingly, the narration in 2400 A.D. is bits and pieces, driven forward by clues, often insignificant by itself, that you should -- though not necessarily must -- collect. Sometimes a single clue, correctly understood, may get you through the puzzle; sometimes even three of them are not enough. Even strictly adhering to the main quest line is not always the best option! Which is only one of this game's many surprising aspects.