BECAUSE I am a free-lance chef whose bailiwick is rural Maine, I have to make do with what's at hand more frequently than most cooks. But what happened to me could happen to you. A four-foot fish does turn up from time to time, even in these urban environs, and the urge to astound one's dinner guests is nearly universal.

As usual, I was catering a wedding dinner 20 miles from nowhere. When I arrived, I already knew quite a bit. I knew there were going to be 70 guests. I knew I was making, among other things, cold poached salmon with green mayonnaise. And I knew that the hostess, herself a skilled cook, had already procured the salmon: "Five large ones, or six if they're small."

I knew wrong. There weren't five or six salmon, there were two.

They were, according to my employer's note, "on ice in the laundry room sink." The ice was in the sink all right. The salmon (of which the smaller measured well over a yard) appeared to be pretty much everywhere.

The ovens were too small. There was no pan in the kitchen, no washtub in the laundry room big enough even for junior.

Any sensible caterer, any sensible cook, any sensible human being, if it comes to that, would have bifurcated those fish faster than King Solomon, poached them, then reassembled the halves. Lots of green mayonnaise at the seams and nobody the wiser -- to say nothing of more evenly cooked fish.

And yet I hesitated. Those fish were so gorgeous, so noble, so challenging in their unwielding entirety.

Their purchaser, who had by this time returned, agreed. All well and good, but now what? Build a giant barbecue and roast 'em over the coals? Poach them in the washing machine, set on "gentle" and half-filled with wine?

Just as we were about to despair, I remembered about the Indians. According to my old Brownie troop leader, the Indians achieved moist cookery by dropping hot stones into hollow logs filled with water. Could we not do the same? A bathtub is not unlike a hollow log, after all, and stones are not yet rare.

A son of the house went down to the seashore to get a supply of stones, while a helpful neighbor started cleaning an upstairs bathtub.

There was a much better bathtub downstairs, a bathtub I would have preferred for its small size and proximity to the kitchen. But it had been decided, in retrospect and quite reasonably, that house guests who have just made a thousand-mile journey do not want to discover they have been assigned a steaming bathtub afloat with several vegetables and two very large fish.

Too bad. The downstairs bathtub was easy. Never try to cook a big fish in the upstairs bathtub is my advice to you.

By dinner time, the salmon (sturdier than the staircase) were glorious. They were massive and exquisite and well-wreathed-around with flowers, and they were, though I say so myself, delicious.

HOW TO COOK A SALMON IN THE BATHTUB

Begin, naturally, by cleaning the bathtub. It must be porcelain. You can't beat on the modern fiberglass jobs hard enough to really clean them.

Start with strong cleanser, then water mixed with vinegar in a 4-to-1 ratio. Finally, rinse down everything with plenty of boiling hot water. Fill the tub with hot tap water and let it sit, replenishing the water as needed, until the tub is thoroughly heated.

Stuff the fish loosely with pieces of lemon, garlic and aromatic herbs. This is partly to impart nice flavors and partly to keep the top and bottom of the fish somewhat separated, so the heat can penetrate more easily. Swaddle the fish with cheesecloth, to help it keep its shape, and tie it securely in two or three places with kitchen string.

Put the fish on a heat-proof, rigid support -- a hardwood board, a large metal tray, a section of the bookshelf wrapped in foil. It has to support the weight of the fish, but it needn't be exactly fish length, particularly if you've been thorough -- nay, Egyptian -- in the matter of cheesecloth-wrapping.

Meanwhile, the rocks are heating. Use sound bricks, granite paving stones or whatever you can get that isn't shale, which might explode. Get enough to half-pave the tub. Start them in a cold oven, turn the heat to 200 and bake for 20 minutes. Raise the heat to 375 degrees and give them another hour or so, a bit longer if they're really big. Stones that weigh much more than five pounds are not such a red-hot idea, because you have to carry them when they're red-hot.

Meanwhile, you are also heating several big pots of water which contain such condiments as parsley, bay leaves, peppercorns and, if you are wealthy, wine. You need enough boiling poaching liquid to come about halfway up the fish.

Drain the tub and place the fish, on their support-board, therein. Carefully, so you don't bust the tub, lower in the hot rocks wrapped in (detergent-free) dish towels for easier transport. Gently tug away the towels, so they don't soak up all the poaching liquid. Try to keep the rocks close to the fish without letting them actually touch.

Pour in the boiling liquid; the fish should be half-submerged. You can top things off with hot tap water if you absolutely must.

Use a long-handled saucepan to ladle the steaming liquid over the fish. Concentrate on the mid-section; the steam will take care of the tail end and nobody's going to mess with the head much anyway. Keep ladling; a fish six or seven inches thick may take an hour or more.

When you think the fish is done, use a razor blade to make an incision through the cheesecloth at the thickest part of the backbone and take a look. The flesh should be semi-opaque, the last quarter-inch next to the backbone not yet quite completely cooked.

Drain the tub. Locate a serving platter. (We used the marble top of a hall table.) Locate someone to help you lift the fish and its support board. It was heavy before; now it's heavy and hot and wet, so if you can let it cool in the bathtub, so much the better. Regardless, let it cool to lukewarm wrapped in the cheesecloth, then transfer it from cooking to serving platter. If you film both platters with cold water, the fish will slide around obediently and you can put it where you want it. It is easier to cut away the cheesecloth than it is to unwrap the fish.

From there on, it's plain sailing, no different from any salmon in aspic except you won't need as much aspic as you'd think.