Fighting to hold back the forces of evil is a mysteri- ---------------------------------------------------------------------

H. Bruce Franklin's most recent books are ''Robert A. Heinlein: America as Science Fiction'' and ''Prison Literature in America: The Victim as Criminal and Artist.'' ous organization controlled entirely by the iron will of ''the Old Man,'' Friday's crusty ''Boss,'' Hartley Baldwin. Friday, despite her mental powers, which dwarf those of the most advanced computer network of this supertechnological future, never discovers this man's true identity. But Heinlein readers soon recognize him as none other than ''Kettle Belly'' Baldwin, leader of the underground organization of genetic supermen who defeated Communism and seemed destined to replace Homo sapiens in Mr. Heinlein's 1949 novella, ''Gulf.'' Being one up on a superbeing is one of the rewards you get for the price of admission.

Kettle Belly is still pontificating that ''geniuses and supergeniuses always make their own rules,'' the same words that he, and other Heinlein wise men, have been using for decades. But Baldwin, perhaps speaking for Mr. Heinlein, now renounces the belief in salvation through a genetic elite, ruefully admitting, ''When I was younger, I thought I could change this world.'' He instructs Friday, when she is to make her inevitable emigration to another planet, not to choose Olympia, where ''those self-styled supermen'' went. Yet Baldwin has had Friday designed mainly from the genes of two of the supermen who did save the world in ''Gulf'' - a secret courier and a professional assassin.

Besides many references to Mr. Heinlein's oeuvre, there are lots of other half-hidden meanings lying around. Friday herself suggests her namesake, Frigg, the Norse goddess associated with marriage, motherhood and sexual license - the ultimate gal Friday.

The bulk of the novel describes Friday's amours. Unfortunately, Mr. Heinlein has a knack for the difficult task of making sex boring. Neither Friday's sexual partners (as in effective romance) nor the details of her sex life (as in effective pornography) are of interest. Her numerous partners, male and female, are all interchangeable, and the details are coyly vague, unlike the precise descriptions of her sexy clothes, elegant meals and artful fighting techniques. Some readers may decide to skip the sex to get to the good parts.