You are an assassin sent from the future to save the world from invasion. Kill your targets and get to the portal to complete each level. Try to avoid getting hit and complete the mission without being seen. The portal will only appear for a short period of time so make sure to get to it as quickly as possible in order to finish the level. Some levels will have guards that will be searching for anything unusual. Be stealth and avoid raising any alarm. You cannot kill them. You can only kill your target.