The tale opens with a high-society sting carried out by a glamorous assassin, Irma Eckman, Elke Sommer. Dressed as an air stewardess, she slips a booby-trapped cigar to oil magnate Henry Keller, Dervis Ward and escapes by parachute as the private jet erupts in a deadly explosion. She is soon picked up by a sleek speedboat steered by her partner in crime, the stunning Penelope, Sylva Koscina, who continues to move the chessboard of this corporate conspiracy.
The duo then eliminates David Wyngarde, John Stone, in a way that suggests a spear fishing accident, sowing suspicion while leaving no trace of their involvement. Sir John Bledlow, Laurence Naismith, one of Phoenician Oil’s directors, grows convinced that foul play is gnawing at the company’s inner circle after receiving a urgent message Wyngarde had hoped to deliver to Keller about a “matter of life and death.” He enlists Hugh “Bulldog” Drummond, Richard Johnson, to dig into the murky web of corporate ambition and murder.
A mysterious intermediary has proposed to Phoenician Oil a swift merger that would enthrone control within six months for a cool one million pounds. Irma shows up at a board meeting to push for acceptance, but the directors are split, with Henry Bridgenorth, Leonard Rossiter (a strong voice against paying), leading the opposition in a razor-thin five-to-four vote. That night, Irma and Penelope return to Bridgenorth’s apartment, turning a hopeful deal into a fatal strike. When the board reconvenes, they vote unanimously to pay.
Wyngarde’s servant Carloggio, George Pastell, delivers a fragment of a taped message Wyngarde had recorded; the assassins silence him to keep the rest of the message missing. Penelope then delivers a box of deadly cigars to Drummond’s flat while he is away. Brenda, a young woman Drummond’s nephew Robert, Steve Carlson, has brought back to the flat, narrowly escapes the same fate Keller suffered. That night, another attempt on Drummond’s life complicates the investigation.
The following day Irma makes Phoenician Oil another offer: to secure the Akmatan oil concession, despite King Fedra’s entrenched opposition, for another million pounds. Drummond quickly realizes that the garbled tape Wyngarde tried to share referred to the King’s assassination as the pivotal target. Meanwhile, Penelope abducts and tortures Robert, though he cannot reveal anything useful. Drummond tracks Irma to their flat and manages to extract Robert just before a bomb left behind by the two assassins destroys them all. Drummond is shocked to learn that Robert is an old college friend of Akmatan’s King Fedra, Zia Mohyeddin.
Irma then eliminates Weston, another Phoenician board member, Marianne Stone. Drummond heads to the Mediterranean coast to warn King Fedra about the looming threat. He is invited to a castle owned by Carl Petersen, the financial genius behind the assassinations, Nigel Green. The shocking revelation comes soon enough: Petersen is Weston in disguise, a twist that binds the puppeteer to the puppets and raises the stakes for Drummond.
Drummond discovers that his captor’s plans extend far beyond a single merger; Petersen’s machinations are designed to lock in control of Akmata’s oil, with Grace, one of Petersen’s women, Suzanna Leigh, hinting at her desire to leave the operation. Penelope — who has been acting as Drummond’s minder under Petersen’s direction — spends a night in his bed, revealing a complicated web of loyalties and betrayals.
A chessboard of power unfolds as Petersen challenges Drummond in a literal game of strategy, complete with giant motorized pieces. Grace confesses her involvement in Petersen’s scheme, and Drummond learns that the bomb aimed at the King might be carried by Grace herself. In a brutal confrontation, Drummond kills Petersen’s bodyguard Chang, Milton Reid, and hurls Petersen into the very trap used in their chess match.
Drummond and Robert race to the King’s yacht, capturing Irma and Penelope along the way. When Irma and Penelope refuse to disclose the bomb’s location, Drummond searches Grace, stripping her and removing the threat by throwing her overboard. The pair slip away, while Irma and Penelope flee in a speeding boat. As the pair escape, Irma reveals the bomb is hidden in Grace’s hairclip, a detail that shocks Penelope into realizing she had stolen the very object that could destroy the King. The two assassins meet their end when the explosive detonates, and Drummond and Robert plunge into the sea to rescue Grace.
