An international debt collector, a professional femme fatale and an English widow. How are they all connected

An international debt collector, a professional femme fatale, an English widow. Five severed heads found floating in the English Channel, another on a French beach. The brutal murder of a Mancunian gangster. A shadowy organisation who nobody knows the name or or their reason d'etre. How are they all connected? That is what the French and British law enforcement have to find out if they're to prevent a potentially world changing crime.