I've done a little bit. Only really have the player moving and a working gun script, and designs for 3 enemies. The plot for my game is an alien race has sent a signal to Earth threatening war, and after failed peace talks and miscommunication, send alien warriors down to fight. A top ranking soldier is sent to kill the aliens that have been sent down. This is the first jam where I've thought of a good idea, and my first 2D game, so I'll see how it goes.