If Dr Moffat was surprised by the request, he did not show it. Ushering Mma Ramotswe into his study at the back of the house, he took a sphygmomanometer out of his desk drawer and began to wrap the cuff round Mma Ramotswe’s proffered arm.

“Have you been feeling unwell?” he asked quietly as he inflated the instrument.

“No,” said Mma Ramotswe. “It’s just that I needed to know.”

Dr Moffat looked at the mercury. “It’s a tiny bit higher than would be wise,” he said. “It’s one hundred and sixty over ninety. In a case like that we should probably do some other tests.”

Mma Ramotswe stared at him. “Are you sure about that reading?” she asked.

Dr Moffat told her that he was. “It’s not too bad,” he said.

“It’s exactly what I thought it would be.”

He gave her a curious look. “Oh? Why would you think that?”

She did not answer the question, but reached into her pocket and took out the bottle of pills which Dr Lubega had sold her. “Do you know these pills?” she asked.

Dr Moffat looked at the label. “That is a well-known pill for high blood pressure,” he said. “It’s very good. Rather costly. But very good. It’s a beta-blocker combined with a diuretic.”

He opened the bottle and spilled a couple of pills out onto his hand. He seemed interested in them, and he held one up closer to examine it.

“That’s a bit odd,” he said, after a moment. “I don’t remember this drug looking like that. I seem to recall it was white. I could be wrong, of course. These are … blue, aren’t they? Yes, definitely blue.”

He replaced the pills in the bottle and crossed the floor of his study to reach for a volume from the bookshelf. “This is a copy of the British National Formulary,” he said. “It lists all the proprietary drugs and describes their appearance. Let me take a look.”

It took him a few minutes to find the drug, but when he did he nodded his agreement with what he read. “There it is,” he said, reading from the formulary. “White tablets. Each tablet contains fifty milligrams of beta-blocker and twelve point five milligrams of diuretic.” He closed the book and looked at Mma Ramotswe over the top of his spectacles.

“I think you’re going to have to tell me where these came from, Mma Ramotswe,” he said. “But would it be easier to do so over a cup of tea? I’m sure that Fiona would be happy to make us all a cup of tea while you tell me all about it.”

“Tea would be very good,” said Mma Ramotswe.