With Leni leaning over him eagerly, Lockhart unfolded the blueprint, revealing a detailed map of Mittelbau. It appeared to consist of two parallel tunnels, each over a mile long. These were connected by cross-tunnels, each around six hundred feet long, and numbered from 1 to 46. These were clearly the halls in which the rockets were assembled. Lockhart quickly scanned them—number 20, for example, was labeled “Tolerance inspection,” number 38 “Assembly of ailerons/tail section.” It was not clear what halls 43 to 46 were for—Lockhart guessed this was where the chemical warheads were fitted.

As he studied the plan, he felt his eyelids beginning to droop. He was absurdly tired. But there was no time for sleep, not now. What he needed to find out, more urgently than anything else, was how a gas that could kill millions could be safely destroyed. It came to him a few minutes later, as he lay on the bed with Leni. He kicked himself that he hadn’t thought of him before. The Test Tube, that was who he needed. He’d know, and—what was more—he was in Germany.

Oflag IX A/H was a castle high above the town of Spangenburg, about forty-five miles south of Hildesheim. It housed Allied officers, some of whom had been POWs since 1940. Among them, Lockhart had got Strasser to discover, was Barham, Lt. Edward, Royal Artillery, POW number 1197. Why did Lockhart want to go there? Strasser had asked. Because Lieutenant Barham was an old friend and he wanted to see him for a drink. Lockhart could tell that Strasser was deeply suspicious, but, with his hands tied, he supplied Lockhart with the necessary papers, along with a letter to the camp commandant, saying that Hauptsturmführer Lockhart was recruiting for the BFC.

Spangenburg Castle was a grim place, thought Lockhart. A light rain was falling, making the grayness of the castle more gloomy, more forbidding. Poor Test Tube, what a place to have been stuck in all these years. Had he tried to escape? Lockhart remembered that Barham was notoriously clumsy—he doubted whether any other officer would care for him as an escape partner.

Lockhart walked over the drawbridge and presented himself to the guard under the vaulted gateway. He was nodded through, and found himself tripping over the badly laid cobbles, thinking that they would have been obstacle enough for any escaper, let alone Test Tube.

“John? What the hell…?”

“Hello, Ted.”

Lockhart stood up and shook Barham’s hand. After much cajoling by Lockhart, the commandant had given them ten minutes alone and the use of a small interrogation room in the German side of the castle. The furniture consisted of nothing more than three rickety wooden chairs and a solid table, upon which Lockhart had placed a packet of cigarettes.

“But what are you doing here? And that uniform! Don’t tell me you’re…”

“No, Ted, I’m not. This isn’t what it looks like.”