- When Harriet Zapper is hired to find the identity of Karel Duval, and the true heir to the Duval family fortune, a bloody trail of murders leads the Big Zapper to the South of France and into a hornet's nest of betrayal and death.
- Two men drive in their Rolls Royces through a a narrow road in the middle of a rural English plain, each in the company of a driver and a companion in tuxedos. They stop at a lonely place obviously set up in advance, and the men compliment each others with short and dagger they had brought in the boxes of their cars. Then, followed only by the two witnesses, they walk further away from the road. They fight a duel, and the man in a black shirt wins first blood on the blond man in a red shirt. The man in black keeps dominating the fight, and inflicts a couple more wounds on his opponent - until he has him at his mercy. The blond man demands grace, and promises never to visit France again in exchange for having his life spared.
Renaud Duval (ALAN LAKE) reproaches his son Christian Duval (Noel Johnson) - the man in black - for his continuing pursuit of silly duels, and says that he hopes him to help with the education of his brother, Karel (JASON KEMP). Renaud then asks him to sign a testament in favor of Guy Champion (EDINA RONAY), a young blonde assistant and Renaud's lover. As the older man refuses, he menaces him with his foil, and the man ends up signing the document. Next, Renaud kills his father, and takes the family medallion from him, to place it around his own neck.
He signs Zendor with the man's blood on a piece of paper, with the typed message, «Revenge is sweet. Your young brother Karel is next.» Then, he impales the piece of paper in his foil, and again skewers his late father with it. He waits the boy to come from college to give him the news, and tell him that Karel is an adopted son, and that all the manor's estate was left only to Renaud. Renaud shows how charitable he is by promising to take care of him for life, and tells the boy he had been found on the street, so he has no idea who his real father was.
Meanwhile, Harriet Zapper, private eye (LINDA MARLOWE), is at home waiting for someone to need her services. She is in the company of Hock (TONY THEN), a kung fu maniac. They are interrupted by Karel, who wants to hire her services to search for his real father.
Guy Champion is actually a cross-bow champion, able to place six bolts in the center of a target. Her next target is Karel, when the boy is rowing alone in a river next to his college. He falls on the river with the impact of the bolt, but is able to shout for help - and by chance, there was a blonde girl around to save him.
Karel meets with Zapper again, and this time, he tells the details of his father's murder - slashed by Alex Zendor, a friend of his brother.
Zendor practices his fencing with Guy, and shows his expertise disarming her first, stripping her nude with a single thrust of the foil, and then cutting part of her hairs that cover her sex. She's thrilled by his action. Renaud presses his brother to duel with him - giving a foil to Karel, and using a heavier sword himself. The duel is interrupted by Zapper, who wants to interview Renaud. The boy claims to be tired, and wants to stop, but Renaud cuts off a bit of blonde hair and presses on. He seems about to impale the boy, when Zapper reaches him. Renaud shows his ability by lighting a match with a slash of his sword, when she was going to smoke.
She lives the manor house with Karel, with the false pretext that her car is not alright, and on her way she stops at a road side bar. She passes a phone call to Claude, asking him to meet there immediately, hiring his services to be bodyguard to Karel while she goes to France in her quest for the truth. Waiting for her, Karel is entertained by five dancers lightly dressed, on a rehearsal conducted by the Compere. Next, she hires a Bar-fly (Graham Ashley) for a strange scene - and acts out a false sexual harassment scene, just in time for Karel to be seen punching the Bar-Fly by a Detective and a Bobby, who arrest the boy, accused by Zapper of molesting her - thus putting Karel away from his brother's reach.
In a gym, Zapper is menaced by six bodybuilders (Bob Prowse, Bruce Ferguson, Colin Osborne, Howard Nelson, Terry Mundy, Chris Roberts), and one of them punches her face until she is KO. Her Chinese side-kick Hock, whom she had given the address of the gym, arrives then, and knocks all of them out with kung fu, starting with the bigger man (Howard Nelson) who had beaten Zapper.
Revived, Zapper goes straight to the airport with Hock, and leaves him behind to pursue the inquiry on Renaud, as she goes to the south of France to search the birth registrars. However, since she rents a car at Nice's airport, she is followed by Guy Champion on another small white car. Zapper heads to Canne's city hall, where she is received by a Gendarme (Michael O'Malley) - more interested in eating a baguette over 1 meter long than in attending to her - and Inspector Cook (William Ridoutt) - who chooses to clean up his pistol while talking with her at the hall. She is informed that the register of Antibes she wanted to consult is at the city hall of Cannes or rather, it was, because it was burned down in a fire that very morning. The Inspector is useful for finding her the whereabouts of a man named Rabellais... by looking it up at the telephone directory book. He lives in a yacht - where apparently he has a fixed telephone line...
Zapper drives to Cannes, but just after the junction to the La Tour de l'Esquillon hotel restaurant, she has to stop to ask the way from a local by the road side. Guy takes the opportunity to run up a hillock - on high heels and mini skirt - adjust her bow and throw a bolt at Zapper's neck. Just as Zapper turned round, thanking the man, got in the car and road on - not noticing that the man had taken the dart right in his chest, and was falling dead on the road.
Hock enlists Renaud's fencing school, and pays with blood his poor defense, as Renaud takes pleasure in seeing the blood squish from his opponent's bodies.
Zapper locates Rabellais (Peter Halliday) easily, and in a promenade along the Bay of Cannes, Rabellais informs her that Karel is the only son of the deceased Duval, Renaud being actually a child he had taken from the streets, and adopted.
For no apparent reason, Guy makes a phone call to Renaud to tell him that her attempt on Zapper's life had failed; this has two effects, enraging him, and stopping him from further injuring Hock with his sword.
Zapper decides to stop her car and go for a swim, taking her revolver with her wrapped in a plastic bag. She is being followed by Guy (EDINA RONAY), in mini skirt and boots, who strides into the water in pursuit, firing crossbow bolts at her. Zapper keeps diving underwater to dodge the bolts, much to Guy's frustration. Then Zapper suddenly leaps back up again close to Guy, with a gun in her hand. Guy looks shocked, then Harriet pumps several bullets into her, and Guy falls backwards with a moan into the sea, where her corpse floats in the wavelets.
Next morning, Zapper visits Rabellais at his mansion overlooking the sea. He gives her a letter by Duval, reading: «The map attached to this note reveals the whereabouts of a small box that I will bury on my estate in England. I will leave in this box the answer to my true will and testament and all the documents relating to my two children together with a key that will open my safety deposit box in the Bank of England.» The drawn map has its own explanation: «North six paces from urn towards tree turn then three paces turn three paces north».
Zapper flies back to England, and meets Hock at home, where the young man is practicing with the foil alone, with no sign of injury on his right upper arm... Zapper takes Karel out of jail and tells him the truth about his birth.
Hock jumps the high wall to gain access to the manor house of Renaud. He tries to follow the instructions on the map, starting from a large decorative urn in the garden of the house. He finds easily the little box. As easily as Renaud finds him. He is forced to hand the paper in the box to Renaud, who reads it to himself: «Embedded in the stone of jade that I wear around my neck is the key to my safety deposit box containing all the documents to my sons and my final will and testament. (a) Christian Duval»
Renaud eats the paper before Hock could read it. Then he forces Hock to a duel... "to show what you're able to do," Renaud says.
Zapper arrives at the mansion with Karel, but only after Renaud kills Hock with sadistic refinement. She starts talking to him inside the house - but then leaves the two brothers alone, as she wants to meet with Hock.
The First Swordsman (Peter Wonson) warns Renaud that the Chinese man is dying in the garden, which he can't believe because he had left him for dead a minute before.
Zapper leaves the house to go back to the garden, just in time to hear Hock's last words. She pick's Hock's foil and goes back to the house, but is surrounded by the six swordsmen, who take her on with rapiers. She wounds the First Swordsman, and kills three of them (James Hennessy, Michael O'Brien, E. Pollard), one by one.
Meanwhile, Karel is able to run away from home, fencing for his life against Renaud, who injures him several times. Zapper is still occupied with the last two swordsmen (Frederick Marks, Bruce Lidington). Renaud is distracted by the First Swordsman, and while he finishes him again showing cruelty, he leaves Karel bleeding against the car box.
At last, Zapper makes a double kill of the last two swordsmen. Then, Zapper takes on Renaud one-on-one with the naked foils. It's a humiliating defeat to the swordsman, who's tripping over his own feet as Harriett slices him to ribbons with ease.
Karel pleads for his life, "Don't kill him. He's my brother."
And Zapper hands him the little key to the bank's deposit box: "This key is yours, Karel. You're the only son of Christian Duval."
"How is my brother?"
"Don't worry about him."
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