In every Level, your mission is to kill the guy with green clothews then escape.
There are some security officers in every level. They are smart. They will attack and try to kill you if they see you. When you break the line of sight, they will follow you. They won't stop searching unless you stay hidden long enough. You die with several bullet hits.
You should use the environment to sneak around and stay hidden. Don't hurry to kill your target, you should be calm and think fast to succeed. It Can be hard to stay alive once they notice you.
You have 2 special abilities. Speed and Invisibility. They use your energy. Your energy refills itself in time. You can activate speed by pressing Z and invisibility by pressing X. You can use invisibility only when you are not moving.
After you have killed the target by reaching him, you must wait for the mission to complete. Mission won't make progress when they are searching for you (alarm mode). They will start searching for you if someone sees the dead body. When mission completes, an escape gate will appear somewhere in the level. You can't enter the gate in alarm mode.
Levels will get harder and harder as you progress through the game. Your progress is saved automatically. You can continue your game from the same webpage.