I like the story setup at the start as it helps set the mood of the game. The automated firing helps keeps the player moving without having to think too much about precise aiming.
Triggering the automated shooting felt inconsistent, as sometimes the gun would fire immediately on mouse-overing the enemies and other times it wouldn't unless I aim at their heads. A potential improvement could be to increase the trigger area to be slightly larger than the enemies' model, and to have the player model switch to an aiming stance. The aiming stance would also mean that the bullet is firing from the center of the character model instead of from the player's right hand. There also didn't seem to be a detriment to being shot by the enemies - Given that a level is 20 seconds long and that the enemies also die in 1 hit, it may make sense to have the player die on a single hit as well.
I enjoyed the random level layout generation - It adds variety to each run and fits with the "explore" aspect. However, given that enemies were not much of a threat, I felt that the levels felt very short when only finding 3 codes. Would it be possible to vary the number of codes that need to be gathered per generated level as well? I think you may be able to get away with 4 or even 5 codes in a level, and it would add a surprise factor for the player per run.