To add to the confusion, this novel (which is in fact written in the form of a play allowing several viewpoints on one subject) was, er, originally a play. And this play was based on true events that took place in 1949, when pieces of a dead body (apart from the head) were found in various goods train wagons and shortly after linked to one viaduct that all the trains had passed under.
L'Amante Anglaise is in three parts. In the first section an unnamed interviewer speaks to Robert Lamy; in the second the interviewer speaks to Pierre Lannes, the husband of Claire Lannes; and in the third the interviewer speaks to Claire herself.
Lamy is the patron of 'Le Balto' café in Viorne (the place where the body parts were disposed of) and fills in some factual details about an audiotape that was recorded in his café several days after the murder, when the Lannes were present with Lamy, as was Italian immigrant Alphonso, and two strangers, one of whom is an undercover policeman. At the end of the tape the policeman arrests Claire for the murder of her deaf and dumb cousin Marie-Thérèse Bousquer. Marie-Thérèse occasionally had sex with Alphonso, who was probably in love with Claire, and he was one of the few people Claire related to. The interviewer thinks the crime is more complicated than it seems to be, is trying to understand Claire, and can't find a motive for the murder.
The second section is the interviewer questioning Pierre. Factually this is incorrect as Claire had in reality killed him, but Duras changed this because she thought he should be allowed to have his say.
Finally, the prison interview with Claire at first reveals a playful but apparently rational woman who won't reveal why she's killed her cousin, nor where she's put her head. The interviewer's enthusiastic voice begins to disappear as it is revealed, with chilling clarity, that Claire is in fact hopelessly insane.