4 ideas
The narrator, a Polish writer named Joanna, stays with her friend Alicja at her house in Allerød, Denmark, during a crowded summer gathering of Polish and Danish guests. The surviving guests work out that Alicja herself is the intended victim and that the killer must be someone inside their own circle.
When a killer strikes the wrong person, investigating the victim's enemies, debts and secrets leads nowhere, because the victim was never the point. The productive question is who was meant to be standing there. Rebuilding the attacker's expectation of where the target would be, and when, reveals both the real target and who had enough intimate knowledge of the household's routines to form that expectation.
In a crowded, disorderly household, people wander in and out, objects go missing, and nobody can account for anyone else's movements. That noise hides a deliberate actor better than any planned alibi. Evidence is misplaced or cleared away before anyone knows it matters, and witnesses cannot tell a suspicious action from ordinary bustle.
Foreign guests caught up in a crime can understand one another's habits, relationships and hints far better than local police who do not share their language or social codes. The official investigation stalls on translation and cultural opacity. Meanwhile the insiders can reach the solution themselves, even though they are also the suspect pool.