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Campion is called in when Andrew Seeley disappears from Socrates Close, a decaying Cambridge household ruled by the aged matriarch Caroline Faraday. Andrew is found dead in the river and other family members die afterwards, which throws suspicion on the living relatives. The solution is that Andrew, who hated his dependent existence and his kin, staged his own death to look like murder and set up delayed lethal traps inside the house before he died.
A killer can arrange harm that takes effect after he is gone, for example poison planted among routine objects. This lets a dead man keep killing. Investigators who look only at who had opportunity at the moment of death will suspect the survivors, because the real agent acted earlier and is no longer a suspect.
When adults are kept financially and socially dependent under one autocratic elder, their frustrations cannot escape. Instead they build up as petty resentments aimed at one another. A closed household of this kind can turn ordinary grievance into lethal intent, and its rigid routines make it easy for someone inside to plant predictable traps.
The matriarch's rule looks like oppressive tyranny, yet she turns out to be the household's shrewdest and most honourable mind. She cooperates with the detective to protect the family's integrity. The novel holds that a strong elder's discipline can be legitimate stewardship, while the real danger lies in the resentful dependents whom that rule shelters.