FAST FORWARD by Judy Mercer (Pocket Books: $22; 352 pp.) "Amnesia!" says Henry Heller, speaking for us all. "I didn't believe it really existed outside of soap operas." Nonetheless, Ariel Gold has it. Royally. And it is a tribute to first-novelist Judy Mercer that Ariel's loss of memory remains so intriguing for so long. (Not for the whole novel, but what the hell. . . .) Imagine waking up battered and bruised in a ransacked house not knowing where you are. Worse, not knowing who you are. Worse still, discovering that you are fat (you hate fat), you smoke (you hate tobacco), you're "homely as a yam" and you dress funny. On the plus side, learning from Henry (your boss, apparently) that you're a lot more fun than you used to be ("withdrawn, serious, private"), that you have some money in the bank, and that you're a talented and percipient producer of segments for an investigative program on TV. All well and good, but how do you resume a career when your notes are gibberish, your list of contacts meaningless, your computer a mystery? Further, are you married, and if so, where is he? Most immediately, who is trying to kill you and why? Credit Mercer with a rare imagination, the skill and wit to keep plausible a pretty far-out premise and a Machiavellian penchant for plot. While she doesn't entirely avoid the novice's nemesis--"He poured a crystal tumbler of amber liquid"; "She verbally regurgitated all she could remember"; a cop who says "anyhoo"--she does a lot better than most tyros. A good yarn, well told.