When Challenger discovers the wreckage of a World War I transport plane, he's captured by its pilot, Lieutenant Drummond. Drummond has been stranded for five years since the crash, protecting the plane's cargo—a crate of iridium ingots stolen from the Royal Navy. Challenger is astounded to see the iridium because it had been intended for his secret wartime research. The sight of it prompts him to remember the night five years earlier, when he was questioned about the theft by two British intelligence agents working for MI5.

Back on the Plateau, when Challenger and Drummond arrive at the Treehouse, Marguerite recognizes the iridium, too—she also was a suspect in its theft, and she recalls her own MI5 interrogation, the same night Challenger was questioned.

Amazing twists and turns. We find out that there may be more than just coincidence that has brought our small group to the plateau.