At Chateau Frontenac, a body cemented in a bath launches Céline on a new investigation.

The room in which a discovery of a person named Philippin was found was rented by a woman named Esther Smith.

Nathan knows Sylvie and the Prévost family well; Daphne discovers that to avoid being removed from the case, Céline has withheld information from her linking William to Esther Smith.

The details surrounding the autopsy of Esther's body reveals that she was thrown overboard from the ferry.

Céline and Daphne discover that Esther was an engineer specialising in large structures. With a committee of experts, Esther was in charge of evaluating the options for the construction of a third link between Quebec City and its southern shore.

Céline, supported by Alex, discovers that Philippin was developing a high-performance concrete. Enough to crush the competition: solid, durable, and 10 times less polluting than the others on the market. A discovery that would be worth billions.