The story in Simon Hunt's one player, joystick-operated sequel to Diamonds is that Brian the Blob has stolen the Great Diamond and stashed it in "the deepest vault". In other words, your little person has to penetrate six maze levels and then get out again. Brian chases you relentlessly while various hieroglyphic nasties pass up or down on seemingly random courses.
To get from one level to another you have to clear each level of mushrooms or small diamonds and it gets harder (what else?) as you go down.
Not exactly mould-breaking or earth-shattering but good fun, with some nice touches. The Level 3 spider's web are very well done, as is Dan's little ghost when he gets killed. There are times, particularly round the randomly opening and closing doors on two of the levels and places where you can't go up, when you get killed more by bad luck than bad judgement.
I think three lives, with no bonuses, is a bit mean, and certainly makes it hard to get into the game.