Bethel was explaining something technical to Tony when I broke in.

"This place is as dry as a bone, Tony; there have been no leaks recently." I turned to Bethel.

"When did you last strip down any of this?"

He frowned.

"Must have been eight months ago, Mr. Mangan. A normal maintenance check. This equipment is efficient; hardly ever goes wrong."

"Where does the water come from?"

"Out of the mains supply." He nodded towards Mackav.

"Mr. Mackay can tell you more about that."

"Then why should we be the only building hit?" I asked Tony.

"That's not exactly true," said "Mackay

"Isn't the mains water piped into tanks somewhere at the top of the building?"

"That's right," said Bethel.

"Right at the top to give it a good head."

"So it could have been contaminated in the tanks after it left the mains," I said.

"I don't think it could have happened down here.

Everything is as tight as a drum. "

"Let's go to the top," proposed Tony, so we went up in the service elevator.

The water tanks were on the roof and they were big.

"Twenty-five thousand gallons," said Bethel.

"Five thousand in each tank." He pointed out the mains piping rising up the side of the hotel.

"The water comes up there and is distributed by this manifold into the tanks. Each tank has a ball valve to control the water level." He shrugged.

"The whole system is just the same as the one you'll have in your own home; it's just that this is bigger."

"I've never seen mine," I said.

Bethel grinned.

"I don't come up here too often myself. The system is automatic." He pointed.

"You can see that the tanks are all interconnected by that manifold at the bottom."

That meant that water would flow freely between the tanks.

"Why five?" I asked.

"Why not one big tank?"

"Well, if something happens a tank springs a leak, say we can isolate it and go on using the other four." Bethel was very good at answering stupid questions from a layman.

"And the tanks are sealed?"

"Sure. There's a manhole on the top of each so we can get at a sticky valve if we have to, but the lids are bolted down on a mastic seal."

"Let's take a look," said Tony, and began to climb the steel ladder on the side of the nearest tank.

We all followed him. On top of the tank Bethel squatted on his haunches.

"Here's the manhole. I had the tanks repainted about three months ago and we just painted over the manhole covers, bolts and all. You can see this hasn't been opened since then the paint seal isn't cracked."

I looked at Mackay.

"Then how did the bug get into the system? It must be in the mains water." Something bright on the roof shot a sun reflection into my eye and I turned slightly to get rid of it.

"Impossible!" said Mackay positively.

"Not if this is the only building affected. Look." He unrolled the chart he was carrying which proved to be a water distribution map.

"All those houses take the same water. Even the airport is on the same water main."

"People normally don't shower in airports," said Tony.

"They do in houses," retorted Mackay.

"It can't be in the mains water. Of that I'm certain."

Bethel had wandered away and was standing on the next tank.

"Hey!" he called, and again the reflection stabbed my eye as I turned.

"This one's been opened." We crossed to the tank and stood around the manhole cover.

"The paint has cracked around the bolts."

"Opened some time in the last three months," said Tony.

"Later than that," said Bethel confidently. He pointed to where bright metal showed where paint had flaked away.

"It hasn't started to rust. I'd say some time in the last week."

"That adds up," said Tony.

"Who would have opened it?" I asked.

"I didn't," said Bethel.

"Harry Crossman might have, but if he did he didn't tell me."

Crossman was Bethel's assistant.

"It will be on his work sheets," I said.

"I want to see them. I want to see them now."

Bethel stood up.

"They're in my office."

"Bring back a wrench," said Tony. T want to take samples from here. "

There was no point in me watching Tony take samples so I went with Bethel. We climbed down on to the roof and walked towards the elevator motor housing, and I kicked something which rolled away and came to a stop with a clink at the edge of a water tank. I stooped and picked it up and found the object that had been sending reflections into my eye.

But it was more than that much more. It was a cylindrical glass tube broken at one end. The1 other end was pointed as though it had been sealed in a flame, and I had seen others like it in Jack Kayles's first-aid box on My Fair Lady. Suddenly ideas came slamming into my head so hard and so fast that they hurt. Whole areas of mystification suddenly became clear and made sense; a weird and unnatural sense, it is true, but conforming to logic.

I turned and yelled, "Tony, come down here."

He clambered down the ladder.

"What's the matter?"

I held out the glass tube.

"Could you take a swab from the inside of there and test it for your damned bug?"

He looked surprised.

"Sure, but…"

"How long will it take?"

"Not long. After the last scare they set up a testing facility in the hospital here. Say, four days."

"I can't wait that long, but take care of it and do your test." I turned and ran for the staircase.

Five minutes later I was talking to Walker at the Royal Palm on Grand Bahama. He said, "Where are you, Mr. Mangan? I'm supposed to be body guarding you."

"I had to leave in a hurry, but never mind that. I want you to send a man on to the roof. No one is to get near the water tanks up there."

"The water tanks!" he echoed.

"What the…"

"Never mind arguing, just do it," I said sharply.

"And put another man near the air-conditioning cooling tower. Nobody is to get near that, either. Nobody at all."

"Not your maintenance crew?"

"Nobody," I said flatly. I did not know ifCarrasco had local assistance or not, but I was taking no chances.

"Where's Carrasco?"

"He spent the day sightseeing in West End," said Walker a shade wearily.

"Right now he's having dinner at the Buccaneer Club out at Deadman Reef. I have two men with him Rodriguez and Palmer."

"You'll probably have police to help you at the hotel as soon as I've talked to Perigord. And after that I'm flying back."

As I rang off Bethel came in.

"Nothing in Harry's work sheets, Mr. Mangan."

"I know. He didn't do it. Do you know Bobby Bowen, my pilot?" Bethel nodded.

"Chase him up, will you? Tell him we'll be flying to Freeport. Oh, and tell Dr. Bosworth he'll be coming with me. Mackay can take the samples to the hospital." Bethel turned to go, and I added, "And thanks. You've been a great help."

When he had gone I rang Perigord. He was not in his office, not entirely unnaturally considering the time of day, but neither was he at home. The telephone was answered by his daughter who told me in a piping voice that Mummy and Daddy were out. Where were they? She was vague about that. They had gone out to dinner. Could be the Stoned Crab or the Captain's Charthouse or possibly the Japanese Steak House in the International Bazaar or the Lobster House in the Mall.

Or was it the Lucayan Country Club? I sighed and thanked her, then reached for the Grand Bahama telephone directory.

I found him in none of those places but finally ran him to earth in the Mai Tai. It took me some time to convince him of my sanity and even longer to move him to action. I think I ruined his dinner.

Tony Bosworth and I walked into the lobby of the Royal Palm and I noticed immediately the two uniformed policemen, one standing by the elevators, the other at the foot of the staircase. I crossed to the desk.