Opening shot: the treetops of Sherwood Forest. Cut to: the archer taking an arrow from the quiver. It whizzes, in the same thrilling footage used in the movie, toward its target.

The difference is that in this commercial, the arrow turns into a spoon and plops into a waiting bowl. "Now, Prince of Thieves is an exciting cereal," intones a stately voice, half Richard the Lion-Hearted, half Don Pardo. "Crunchy, fruit-flavored arrows that always hit the mark."

It's only a 30-second spot, so perhaps there wasn't time to mention that Prince of Thieves is coated with multicolored sprinkles. Licensed cereals, the industry term for these edible spinoffs aimed at grade-schoolers, use lots of colors. Pinkish marshmallow pizza wedges and green marshmallow amphibians in Teenage Mutant Ninja Turtles cereal. Orange, lilac and yellow marshmallow musical notes in the brand-new Bill and Ted's Excellent Cereal. Patrick Farrell, spokesman for Ralston Purina, which manufactures all of the above, explains that "the vibrance helps a lot with the excitement and fun."

It might be argued, though, that much of the excitement and fun is occurring at Warner Bros. and at Ralston. The former, its licensing division anticipating the millions generated by Prince of Thieves merchandise, has a shot at every potential moviegoer wheeling a cart down the cereal aisle. The latter, building brand awareness every time a consumer sees "Robin Hood: Prince of Thieves" or an ad or poster thereof, is spending perhaps a third the normal cost of launching a new cereal. The life span of a licensed cereal, however vibrant, rarely exceeds 18 months. But that's long enough.

This particular twist in the world of modern, everything-promotes-everything-else marketing is largely a Ralston Purina development. General Mills ventured in early with Strawberry Shortcake (she was an icky-poo character swathed in gingham) and Pac-Man cereals in the early to mid-'80s. It also created what seems to have been the first true movie cereal, E.T., with chocolate-and-peanut-butter-flavored E's and T's (perhaps designing aliens in grain was prohibitively costly). But the No. 2 cereal maker switched strategies and got out of licensed products by 1985.

"It's such a short-lived thing," says Kathryn Newton, General Mills' public relations manager. "Stick with the tried and true." The company had its own characters to help tykes remember which box to point to and shriek for, like the Trix rabbit and the Lucky Charms leprechaun, so why profit-share with Steven Spielberg? (Royalty rates on food products typically range from 3 percent to 5 percent of sales.) The fact is that with roughly a quarter of the branded ready-to-eat cereal market (in tonnage, not dollars), General Mills doesn't need Robin Hood.

Enter Ralston in 1985 with Cabbage Patch Kids cereal. There's been at least one licensed cereal out of Checkerboard Square, sweetened and spun off from film, toy or video game (categories growing increasingly blurry; anything that's one becomes the others), on supermarket shelves ever since. With only a little more than 5 percent of the market -- Kellogg's Frosted Flakes alone has about the same share -- plus another 4 percent in private-label manufacturing for supermarket chains, Ralston could use a hand from someone who robs the rich.

"It's a survival technique," says John McMillin, a food industry analyst for Prudential Securities. "It doesn't work for everybody, but it works for the little guy."

The $7 billion cold-cereal business has high profit margins, but introducing a new brand when there are already some 220 vying for shelf space and consumer recognition has become a major investment. Advertising and marketing expenditures of $35 million to $40 million in the first year are not uncommon. But using popular movie or TV characters lets cereal makers use what marketers call "borrowed interest," capitalizing on a craze someone else created.

Ralston will not provide budget figures, but Polly Kawalek, director of marketing at rival Quaker Oats' ready-to-eat-cereals division, estimates that Ralston spends a mere $10 million to $12 million to launch its licensed products. She adds that Quaker will wind up spending about the same amount introducing its new entry, Tiny Toon Adventurers, which piggybacks on the popular cartoon series and hit the shelves in January. Quaker has a considerably greater market share than Ralston -- close to 8 percent -- but is still dwarfed by the Big Two, Kellogg and General Mills.

"If your name is Kellogg's and you've got Corn Flakes, you don't have to deal with Ninja Turtles," analyst McMillin points out. But smaller companies need "the savings of not having to advertise Batman" -- Ralston introduced a bat-wing-shaped cereal on the heels of the 1989 film. "They don't have to pay for these huge media blitzes."

For Prince of Thieves, for instance, one TV spot shown for a month on Saturday mornings and in other heavy kidvid time slots, plus newspaper-insert coupons to soften up parents, is all the introductory razzmatazz Ralston deems necessary. The steeper prices Ralston sets for its licensed cereals also contribute to a tastier bottom line.

Not that the strategy always works. Quaker may owe its caution in entering the licensing fray -- Tiny Toons is not seen as the first in a series -- to the lingering aftertaste of Mr. T cereal, which failed to reach hoped-for levels of profitability. Similarly, the Jetsons movie released last year was a fizzle and so was the Ralston cereal, shaped like moons and meteors and driven from supermarkets in just 10 months. Breakfast with Barbie fared little better. In fact, many of Ralston's licensed cereals -- Nintendo, Batman, Hot Wheels -- have faded after 14 to 16 months.

But all it takes is one smash like Ghostbusters, whose unprecedented five-year run may have less to do with its ghost-shaped marshmallows than with the sales push of two hit movies and a popular Saturday cartoon, to keep the marketers trying. With the Turtles, just entering Year Three of cerealhood, they've found another winner.

(Footnote: Prince of Thieves cereal might have been even more exciting and fun had the front of the package featured Kevin Costner instead of a generic archer. The star had likeness approval, however, and declined to grace cereal boxes, participate in Taco Bell promotions or be pictured on any Prince of Thieves merchandise except Kenner's action figures.)

That much of what a child eats, wears, rides, plays with, sleeps on and trick-or-treats in can be a product promotion comes as no shock to parents. It's such established practice, in fact, that even a dogged campaigner like Action for Children's Television can't rouse itself to much indignation. "There's so much merchandising targeted to children, so many programs that are in effect commercials, so many commercials finding their way into classrooms, that the idea of having children's cereal commercialized is rather benign," says ACT President Peggy Charren.

The nutritional issue has also been back-burnered. Children's cereals are sugary, virtually by definition, but the licensed sort don't seem more cavity-inducing than the rest of the lot. Bill and Ted's is on the high side with 13 grams of sugar per serving, Tiny Toons in the mid-range with 10, both a far cry from sensible Cheerios (1 gram) but not quite as repellent as Fruity Marshmallow Krispies (17). Neither Turtles nor Prince of Thieves currently carries a carbohydrate information label, but Ralston says that future shipments will, showing sugar levels of 11 and 12 grams, respectively.

Anyway, Ralston is completely unapologetic on the sugar issue. "The premise that sweet is bad is just wrong; these are products with nutritional value," argues spokesman Farrell, who wants points for Ralston's having added vitamins and eliminating tropical oils.

Besides, cereal licensing could be worse. The summer's apparent blockbuster, both in box office and in licensing lust, isn't "Robin Hood" but "Terminator 2." Official Terminator tank tops, skateboards, endoskeleton hobby kits and boxer shorts are all coming soon to a retailer near you. But no one's introducing Cyborg Sweeties cereal, with frosted Schwarzenuggets and marshmallow biceps in three pastel shades. Yet.