Mma Ramotswe lowered her cup, very slowly. Her heart thumped within her; she realised that she was at the heart of the matter now, the elusive solution within her grasp. But she would have to sound casual.

"Oh, they did that up in Nairobi too," she said, waving her hand airily, as if these things were of little consequence. "One helped the other. And usually the patients didn't know that they were seeing a different doctor."

The nurse laughed. "They do it here too," she said. "I'm not sure if it's quite fair on the patients, but nobody has realised that there are two of them. So everybody seems quite satisfied."

Mma Ramotswe picked up her cup again and passed it for refilling. "And what about you?" she said. "Can you tell them apart?"

The nurse handed the teacup back to Mma Ramotswe. "I can tell by one thing," she said. "One of them is quite good- the other's hopeless. The hopeless one knows hardly anything about medicine. If you ask me, it's a miracle that he got through medical school."

Mma Ramotswe thought, but did not say: He didn't.

SHE STAYED in Mafikeng that night, at the Station Hotel, which was noisy and uncomfortable, but she slept well nonetheless, as she always did when she had just finished an enquiry. The next morning she shopped at the OK Bazaars and found, to her delight, that there was a rail of size 22 dresses on special offer. She bought three-two more than she really needed-but if you were the owner of the No. 1 Ladies' Detective Agency you had to keep up a certain style.

She was home by three o'clock that afternoon and she telephoned Dr Maketsi at his house and invited him to come immediately to her office to be informed of the results of her enquiry. He arrived within ten minutes and sat opposite her in the office, fiddling anxiously with the cuffs of his shirt.

"First of all," announced Mma Ramotswe, "no drugs."

Dr Maketsi breathed a sigh of relief. "Thank goodness for that," he said. "That's one thing I was really worried about."

"Well," said Mma Ramotswe doubtfully. "I'm not sure if you're going to like what I'm going to tell you."

"He's not qualified," gasped Dr Maketsi. "Is that it?"

"One of them is qualified," said Mma Ramotswe.

Dr Maketsi looked blank. "One of them?"

Mma Ramotswe settled back in her chair with the air of one about to reveal a mystery.

"There were once two twins," she began. "One went to medical school and became a doctor. The other did not. The one with the qualification got a job as a doctor, but was greedy and thought that two jobs as a doctor would pay better than one. So he took two jobs, and did both of them part-time. When he wasn't there, his brother, who was his identical twin, you'll recall, did the job for him. He used such medical knowledge as he had picked up from his qualified brother and no doubt also got advice from the brother as to what to do. And that's it. That's the story of Dr Komoti, and his twin brother in Mafikeng."

Dr Maketsi sat absolutely silent. As Mma Ramotswe spoke he had sunk his head in his hands and for a moment she thought that he was going to cry.

"So we've had both of them in the hospital," he said at last. "Sometimes we've had the qualified one, and sometimes we've had the twin brother."

"Yes," said Mma Ramotswe simply. "For three days a week, say, you've had the qualified twin while the unqualified twin practised as a general practitioner in a surgery near Mafikeng Railway Station. Then they'd change about, and I assume that the qualified one would pick up any pieces which the unqualified one had left lying around, so to speak."

"Two jobs for the price of one medical degree," mused Dr Maketsi. "It's the most cunning scheme I've come across for a long, long time."

"I have to admit I was amazed by it," said Mma Ramotswe. "I thought that I'd seen all the varieties of human dishonesty, but obviously one can still be surprised from time to time."

Dr Maketsi rubbed his chin.

"I'll have to go to the police about this," he said. "There's going to have to be a prosecution. We have to protect the public from people like this."

"Unless…" started Mma Ramotswe. Dr Maketsi grabbed at the straw he suspected she might be offering him.

"Can you think of an alternative?" he asked. "Once this gets out, people will take fright. We'll have people encouraging others not to go to hospital. Our public health programmes rely on trust-you know how it is."

"Precisely," said Mma Ramotswe. "I suggest that we transfer the heat elsewhere. I agree with you: the public has to be protected and Dr Komoti is going to have to be struck off, or whatever you people do. But why not get this done in somebody else's patch?"

"Do you mean in Mafikeng?"

"Yes," said Mma Ramotswe. "After all, an offence is being committed down there and we can let the South Africans deal with it. The papers up here in Gaborone probably won't even pick up on it. All that people here will know is that Dr Komoti resigned suddenly, which people often do-for all sorts of reasons."

"Well," said Dr Maketsi. "I would rather like to keep the Minister's nose out of all this. I don't think it would help if he became… how shall we put it, upset?"

"Of course it wouldn't help," said Mma Ramotswe. "With your permission I shall telephone my friend Billy Pilani, who's a police captain down there. He'd love to be seen to expose a bogus doctor. Billy likes a good, sensational arrest."

"You do that," said Dr Maketsi, smiling. This was a tidy solution to a most extraordinary matter, and he was most impressed with the way in which Mma Ramotswe had handled it.

"You know," he said, "I don't think that even my aunt in Mochudi could have dealt with this any better than you have."

Mma Ramotswe smiled at her old friend. You can go through life and make new friends every year-every month practically-but there was never any substitute for those friendships of childhood that survive into adult years. Those are the ones in which we are bound to one another with hoops of steel.

She reached out and touched Dr Maketsi on the arm, gently, as old friends will sometimes do when they have nothing more to say.

A DUSTY track, hardly in use, enough to break the springs; a hill, a tumble of boulders, just as the sketch map drawn by Mr Charlie Gotso had predicted; and above, stretching from horizon to horizon, the empty sky, singing in the heat of noon. Mma Ramotswe steered the tiny white van cautiously, avoiding the rocks that could tear the sump from the car, wondering why nobody came this way. This was dead country; no cattle, no goats; only the bush and the stunted thorn trees. That anybody should want to live here, away from a village, away from human contact, seemed inexplicable. Dead country.

Suddenly she saw the house, tucked away behind the trees, almost in the shadow of the hill. It was a bare earth house in the traditional style; brown mud walls, a few glassless windows, with a knee-height wall around the yard. A previous owner, a long time ago, had painted designs on the wall, but neglect and the years had scaled them off and only their ghosts remained.

She parked the van and drew in her breath. She had faced down fraudsters; she had coped with jealous wives; she had even stood up to Mr Gotso; but this meeting would be different. This was evil incarnate, the heart of darkness, the root of shame. This man, for all his mumbo-jumbo and his spells, was a murderer.

She opened the door and eased herself out of the van. The sun was riding high and its light prickled at her skin. They were too far west here, too close to the Kalahari, and her unease increased. This was not the comforting land she had grown up with; this was the merciless Africa, the waterless land.