Test Subject Green

Test Subject Green is another well made Nitrome title and a sequel to one of their other games called test Subject Blue. In Test Subject Green you take on the role of a mad scientist's single celled test subject and you are placed in a not so friendly sealed test environment. You have some kind of exo skeleton/suit with a gun to fend of enemies while trying to reach the end point of each level. The levels are seemingly small, but to reach the goal you often need to travel through several portals thus making the whole level a lot bigger. The scientist is monitoring your progress through a window in the test container you're in and the stuff going on outside the container is getting increasingly weird the more levels you complete...

http://www.nitrome.com/games/testsubjectgreen/