Lucrèce is a hired killer. Specialized in the use of poisons, and passionate about opera, she must carry out a difficult contract in the Swiss Alps. Masquerading as a young opera singer, Lucrèce must perform on the stage of the very exclusive Festival d’Ermeux and try to kill one of her partners, the British baritone Alexander Child. He has recently purchased a distillery in Scotland, and remains the only obstacle to a strategic pipeline project that has considerable economic implications. He has just won a drawn-out legal battle with British Oil, which has no other choice but to eliminate him. To complicate matters, the French secret service, informed of this contract, send Rico, once part of the service's Action Division, to infiltrate the orchestra, and to unmask and wipe out everyone who wants to harm Alexander Child. Rico has no information about the killer, and he'll have a lot of difficulty locating her in the middle of this theater crowd, among whom there is an odious conductor, a particularly proud tenor, and a strangely stressed-out festival director. To the rhythm of Handel's sublime Mass, a breathless intrigue begins... that could end in a Requiem... for a killer!