That motive was Natalie Coolidge.

He smiled at the thought, yet confessed it true.

In spite of her curt dismissal, his memory of the girl centred about those earlier hours of their acquaintance.

Something mysterious had occurred to make her change so quickly, and he was unwilling to condemn her before learning the real reason.

This murder must have some relation to the Coolidge estate; he could conceive of no other motive for such a cold-blooded affair; and hence its solving must prove of vital importance to her and her future.
Now, when the verdict of the coroner's jury had been suicide, and when only he, and the servant Sexton suspected otherwise, it was of the utmost importance that they endeavour to unravel the crime.