I've been trying to find a PC game that I played a demo for back in the early 90s (give or take).
It was a space exploration/RPG with sprite graphics and I think a top-down perspective. You fly around in your starship talking to different people and doing missions for them to upgrade your ship and unravel the main plot. I think it was very similar to Star Control 2, unless I'm mashing-up elements of the two in my memory.
The two things I really strongly remember are the storyline and one specific game mechanic.
As you go through the storyline of the demo, it turns out you and the rest of humanity are trapped in some kind of force bubble in space that destroys anything that goes too far from the star. Eventually you find out that it's an intentional prison, and the only way out is a secret wormhole somewhere that lets you escape, where you meet an alien. Immediately after that the demo ends with a message like "Get the full version to play chapter 2!" or some such thing.
The mechanic I recall is having multiple settings for your weapons and shields, each symbolized by a different wave-form symbol (sine wave, square wave, sawtooth, etc). If you used the right beam setting for the enemy's shield setting, it would punch through their shields, or something like that.