4 ideas
In 1930s Helsinki, a wealthy widow, Mrs. Skrof, is found dead from gas inside her closed flat, a death that could pass for a household accident. Inspector Palmu, with his young assistant narrating, works through her relatives, each of whom stands to gain from her money. The case shows an apparently accidental domestic death being reframed as murder and traced through the dead woman's own family circle.
When a death uses an ordinary household mechanism like gas in a closed room, the physical cause is not in dispute. The investigation has to show deliberate arrangement rather than find a weapon. The question changes from who could get in to who could set the conditions and walk away, so small inconsistencies in the scene matter more than dramatic evidence.
When every family member inherits, motive stops pointing at anyone and becomes background noise. The detective has to separate suspects by opportunity, by lies, and by how each person behaves under questioning.
A fair-play detective novel gives the reader every clue the detective uses, so the solution can in principle be reached before it is revealed. Writing within that constraint is proof of discipline. The author has to hide the answer in plain sight through ordering and emphasis rather than by withholding information.