Lucrèce is a contract killer in search of a new life. Specialized in the use of poisons and enamored with opera, she accepts a difficult, final hit in a castle in the heart of the Swiss Alps. Posing as a soprano, Lucrèce must perform on stage during the very prestigious Festival of Ermeux and slaughter one of her co-performers, British baritone Alexander Child, who recently bought a distillery in Scotland, and thus became the only obstacle to a strategic pipeline project with considerable financial ramifications. He just won a long legal battle against British Oil, who now has no other choice but to eliminate him. To complicate things, French counter-espionage learns of the hit and sends Rico, a reinstated former agent, to infiltrate the orchestra, unmask and terminate all who wish harm to Alexander Child. Rico has no information on the killer and has a lot of trouble identifying her amid a horde of suspicious characters: an obnoxious conductor, a particularly proud tenor and a strangely stressed festival director... Throughout rehearsals of Haendel's sublime Messiah, a breathless intrigue begins...and could end in a Requiem...for a killer!