The cheyne mystery

In a country hotel Maxwell Cheyne sits down to lunch with the pleasant stranger he has just met. Then, as the meal ends, he sinks into a drugged sleep and, on waking, learns that his house has been burgled (though nothing is missing)... This is only the first of his encounters with a gang of very persistent criminals. But what are they after? It falls to Inspector French of Scotland Yard to expose their desperate and unguessable conspiracy.