5 ideas
In the title case, a victim is poisoned by a boiled egg whose shell shows no puncture or tampering, which seems to rule out adding poison after cooking. Rolf le Roux, a Cape Town lawyer with psychological training, solves it by questioning the assumption that an unbroken shell proves the contents were never reached. The case is a clean test of the idea that physical seals certify innocence only as far as we understand the physics behind them.
An impossible crime is not a dead end but a constraint. Each apparent barrier, such as a sealed container, a watched room, or an unbroken shell, eliminates whole classes of method. What remains must exploit a hidden assumption the observers made about how the barrier works.
Le Roux combines legal procedure with psychology. He treats how suspects talk, what they avoid saying, and what they assume others will believe as evidence equal to physical traces. The mechanism is that a staged impossibility reveals its designer's mind, because the illusion was built to exploit what the culprit expected witnesses to take for granted.
The stories are built so the reader receives every fact needed to solve the crime before the detective explains it. The difficulty comes from misdirection about which facts matter, not from withheld information. This makes the solution a test of the reader's own unexamined assumptions rather than of their access to data.
Peter Godfrey, regarded as South Africa's leading author of impossible-crime stories, set his puzzles in Cape Town. He later left the country in opposition to apartheid. His case shows a writer using a seemingly apolitical genre that rests on logic, rules of evidence, and justice for the individual, while living under a legal order he found unjust.