Derelict's backstory is straight out of Aliens: you encounter and board a derelict spaceship which proves to be teeming with nasty alien critters who try to eat you and whom you must mow relentlessly down with high-powered, futuristic weapons. But the Alien series is brooding, dark, and bloody, while Derelict is light hearted, well-lit, and rather charming; the aliens may want to eat you, but they're aliens out of Nick Jr. rather than Lovecraft.
In terms of backstory, it's reminiscent of Jim Dunnigan's Wreck of the BSM Pandora, except that it's a shooter instead of a boardgame with a combat results table. In terms of gameplay, however, the obvious analog is Gauntlet. Particularly in the later levels, you are overwhelemed by hordes of teeming aliens, whom you overcome (if you do) with a combination of vastly superior firepower and mad l33t sk1llz. You'd be wading in alien gore, except that, well, Derelict doesn't go there; it's a shooter, but gore is not its thing.
So often, of course, the devil is in the details: is the UI smooth? Is the challenge curve appropriate? Are the levels interesting?
Yes, yes, and yes. Derelict is played with the arrow keys, and three keys for combat (fire, strafe, and special weapon): easy to pick up, even for a player who has trouble with more intense twitch games (like, ah, me). The difficulty ramps up smoothly, so that by the time you're faced with intense battles, you have probably developed the skillz to deal with them; and for a game with a relative handful of elements, confined to the corridors of a spaceship, Derelict manages to provide a remarkably varied array of challenges.
It is, when you come down to it, a fairly conventional (if edge-of-the-seat intense) third-person shooter, with retro (if charming) graphics; but for what it is, it is very nicely executed.