Eric had spotted a barrage balloon earlier when coming out of the underground station. It was one of hundreds flying over London. Barrage balloons might have looked like airships, but they were unmanned, and tethered to the ground. They had netting or cables stretching down from them to a military truck on the ground. They bobbed around in the skies over London to make it more difficult for enemy aircraft to enter the airspace. It meant the Nazi bombers and fighter planes had to fly high over the balloons to avoid them. This made them an easier target for the British anti-aircraft artillery (or big guns) on the ground. If the planes flew too low, then the guns couldn’t spin round fast enough to hit them. Higher in the sky, the guns had much more chance of shooting them down.

“So talk me through this plan of yours,” said Sid.

“We steal,” began Eric, “I mean borrow a balloon, and fly it over the zoo. Once we reach the gorilla enclosure, we open the top of her cage and pluck Gertrude out. Then make our escape across the sky!”

The old man stared into space, lost in thought.

“Uncle Sid?” said the boy. “UNCLE SID! What do you think?”

“I think it’s the least worst idea we’ve had!” he finally replied.

“That means it’s the BEST!”

“Yes, I suppose it is!” said Sid, a flash of worry on his face. “But how do we know it’s going to work?”

“We don’t. Not until we try.”

“Good answer! Now let’s work out how we could fly a barrage balloon.”

Upstairs in his bedroom, Sid had a collection of books about the First World War. In a book on old German war machinery, there was a chapter on Zeppelins. These were the airships used as bombers and scouts in the First World War. Unlike barrage balloons, Zeppelins had engines and a gondola underneath for a pilot. That’s because they were designed to fly, rather than just float in one place.

However, Eric was sure there must be some way of piloting a balloon, perhaps using the truck on the ground to which it was tethered. The problem was that time was running out. He looked at the clock on the kitchen wall. It was now one o’clock in the afternoon. There were only a few more hours until darkness fell. The zoo director general, Sir Frederick Frown, had said that Miss Gnarl was to put Gertrude down as soon as the zoo had been closed for the night, which was five o’clock. If the pair were going to save the poor gorilla from a lethal injection, they had to

act FAST!