It all begins when young, financially embarassed Alan Craig borrows a hefty sum, giving as security a will made in favour of the lenders, Francis Bullard and Robert Lancaster. Craig is about to depart on an Arctic expedition but before he leaves he goes to Scotland to visit his ailing uncle, Christopher Craig, who made a fortune in, and owns a staggeringly valuable number of diamonds from, South Africa.
Time passes but Alan Craig does not return from the Arctic. By then his uncle is dying, as he tells Bullard and Lancaster, who have been friends of his for some time. He also reveals he has willed his fortune to the missing, presumed dead, nephew. But nothing can be done about winding up his estate, including disposing of the diamonds, until the clock stops.
The clock was specially constructed with a mechanism that will stop it a year and a day after it has been started, a task given to a devoted servant to carry out once the master has died. It's a sinister sort of timepiece, for its bottom third is filled with a sluggish green liquid and the niche the clock occupies is labelled 'Dangerous". Is the strange matter gas or poison, explosives, or some sort of corrosive matter? What will happen when time runs out and the clock stops working?
Before the reader discovers the answer to that interesting conundrum, they will have contemplated a veritable Newgate Calendar of crimes including -- but not limited to -- breaking and entering, blackmail, and infernal engines. Toss in romantic entanglements and much to-ing and fro-ing between London and Scotland among other things, and the result is a novel in which the convoluted skein of events eventually works out smoothly and those that deserve it get their comeuppance in satisfactory fashion.