When they arrived there at five o'clock, Quarrier offered Mortimer his hand, and held the trembling, puffy fingers as he leaned closer, saying with cold precision and emotionless emphasis something that appeared to require the full concentration of Mortimer's half-drugged faculties.
And when at length Mortimer drove away in a hansom, Quarrier's Japanese steward went with him--perhaps to carry his suit case--a courtesy that did credit to Quarrier's innate thoughtfulness and consideration for others.

He was very considerate; he even called Agatha up on the telephone and talked with her for ten minutes.

Then he telephoned to Plank's office, learned that Harrington was already there, telephoned the garage for a Mercedes which he always kept ready in town, and presently went bowling away to a conference on which the last few hours had put an entirely new aspect.
It had taken Plank only a few minutes to perceive that something had occurred to change a point of view which he had believed it impossible for Quarrier to change.

Something had gone wrong in his own careful calculations; some cog had slipped, some rivet given way, some bed-plate cracked.

And Harrington evidently had not been aware of it; but Quarrier knew it.