That wasn’t easy for Max to answer, as the answer was “Because we know that someone on this train is smuggling a diamond to Istanbul, and Rupert is one of our suspects, and I eavesdropped on his phone call yesterday so I know that someone is threatening him, and then I was snooping around in the middle of the night after I heard a loud thump and I snuck into Rupert’s room and saw that it was empty”; and that didn’t seem like something she should just announce to everyone over breakfast. There was a longish pause. The radio had moved on to an angsty ballad with lots of dramatic drum rolls.

“Look, Celeste,” began Sister Marguerite, in her best reasonable voice, “we’re all feeling – OW!”

Max had kicked her under the table, but it was too late. Celeste’s eyes had widened in shock; then she rearranged her face into its usual empty stare, snatched up her novel, and swept out of the carriage.

“What?” said Marguerite, rubbing her shin. “What did I do?”

Max was trying to find a way of silently explaining, but it was a difficult sort of thing to mime. How would you say, without words: We haven’t actually been introduced, so neither of us is supposed to know Celeste’s name?

As it turned out, Max didn’t need to explain, because Klaus did the job for her. Well – sort of.

“Oh dear, she’s taken it hard. Oh, poor man. What awful news,” he said. He had hunched his shoulders as small as they would go, and was doing a good imitation of shock and sorrow, but Max could not forget the arrowheads marching across those shoulders under his shirt. After a hunched, sad little pause, Klaus started knitting again, and said casually, “Why did you call her Celeste?” – and there was a moment of penny-dropping on Sister Marguerite’s face, before Klaus confused things, by adding, “Her name’s Suzanne.”

At that, Marguerite was firmly back to confusion again, and this time Max joined her. She made a small beard of confusion out of her plaits. “What? I thought her name was Celeste Le Blanc?”

“Nope,” said Klaus firmly. “I chatted to her on the last train. I like to get to know my fellow passengers.” He paused his needles a moment to tick off all the things that he had learned on his fingers: “Suzanne Leroy, works at the post office in Paris, lives with her boyfriend Paul, two cats and a parrot.” He smiled, pleased with his findings, and resumed knitting.

Max twizzled her plait-beard. Paul? Then who was M? Did it matter anyway, if M had been writing to another woman entirely? She tried to hold all of it straight in her head, but the memory of Rupert’s phone conversation kept getting in the way and making it difficult to think straight.