You have been locked up by The Empire for your rebellious actions. You are a key member of the rebellion that is conspiring to assassinate the emperor and will not be stopped by petty concrete walls. Time to escape.
Bro tips: Don't use explosives on everything you see, many turrets have powerups underneath them for crafty players that beat them without resorting to explosions. Take your steps carefully or enemies will kill you with great efficiency and speed. Read the text if you don't know what to do and don't notice the environmental hints. Take note of the info the Seals of the Empire give you. If the game is too easy then go and play the challenge mode at: http://www.sploder.com/games/members/ravicale/play/imprisoned-challenge-mode/