The opening of film shows a news reporter covering a story in Turkey where many people have mysteriously died, seemingly from pneumonia. The focus then moves to Hong Kong where Buck Yuen (Chan) works as an exercise equipment salesman. After an unsuccessful day at work, he heads out through the shopping mall and intuitively knows that a robbery is about to take place. Buck thwarts the robbers plans, relieving them of the money they have stolen and returning it to the police. Later, a stranger approaches Buck, eager to speak the hero who foiled the robbery. The stranger goes by the name of Manny Liu (Eric Tsang) and he tells Buck that he is rounding up several men of Buck's age and description, one of whom could be the son of a wealthy Korean man.
As a child, Buck had been an orphan, but he has vague dreams seemingly recalling his parents cooing over him, his father clutching a shiny object. Buck goes to meet the Korean man in question, a former spy, who does not have long to live. After fending off an attack on the old man, Buck is given the opportunity to play a "game" with the old man. The old man gives him a crucifix, which appears to be the shiny object of his dreams, confirming this man to be his father. Once the old man has died, Buck finds the first clue to the "game" at his grave - a message saying "wait for me" in English. Eventually he realises that the letters of the phrase correspond to a telephone number and calls it. It turns out to be a bank in Turkey, so Buck sets out on his journey. Once in Turkey, Buck goes to the bank and receives the contents of the old man's safety deposit box - a large sum of money and a small package. Avoiding the attempted theft of his newfound wealth by a group of thugs who have commandeered a fleet of taxis, Buck eventually makes it back to his hotel. Along the way he meets two women - the first, Korean reporter Carmen (Kim Min-Jeong) and the second, a Chinese woman called Yong (Vivian Hsu), who sings sweetly and wears a scarf embroidered with the same phrase from the old man's grave. He catches up with the woman and they arrange to meet later.
Buck then visits a Turkish baths, but is accosted by another group of thugs, and ends up being pursued, wearing nothing but a towel, through a Turkish bazaar. He soon loses his towel, and is forced to hide his nudity with a variety of implements from the various stores, all the while, avoiding attacks from the thugs. He hides in an alleyway and notices huge pieces of cloth hanging from above, so he rapidly performs acrobatic moves to twist the material around himself. Thus disguised in the makeshift clothing he finally manages to make his escape.
Carmen Wong, who had appeared to be a reporter, turns out to be working for the CIA and she informs Buck that the item everyone seems so keen to get their hands on is a new biological weapon, Anthrax II, many times more powerful than regular anthrax. It is this that had killed the many Turkish people in the film's opening scene - Turkey had been chosen as the testing ground. When Buck meets up with Yong, he learns that her boss is crime lord Lee Sang-zen, (Wu Hsing-kuo) and a deal had been brokered between him and Buck's 'father'.
Buck and Yong are then captured by a Turkish gang and tied up, but the gang themselves are subject to an attack from Lee's gang. Buck makes his escape and frees Yong, winding the winch cable of a crane all around the supporting wooden beams of the building and setting the crane to retract the cable. The cable rips through the wooden building, tearing it from its moorings and allows Buck and Yong to escape into the sea. Later the pair are picked up by Lee, and Buck learns that Yong is a drug addict, practically a slave to Lee and her life is in grave danger. Later, as they part, Lee offers Buck a new deal, more money and Yong's freedom if he gives Lee what he wants.
Realizing that if he saves Yong's life, many others may die, Buck seeks the advice of a Turkish priest. The priest knows the various East Asian languages, and was an associate of the old man. He leads Buck to a basement room and gives him the item that everyone has been trying to get their hands on - 2 vials of Anthrax II. He advises Buck to save the one he can and leave the consequences up to God. Buck relents, following the advice of the priest, hands over the vials over to save Yong. However, he soon learns that she has already been given a fatal dose of drugs and is soon dead.
Buck tells Carmen that he gave the vials up for Yong's life and a desperate race to get them back ensues. At the height of the action, the thugs' car becomes lodged into the rear of a large oil tanker, which catches fire. In scenes reminiscent of Speed, the driver is told he must keep the tanker traveling at 80 km/h or else the fire will spread forward, blowing up the tanker and a huge blast radius around it. Buck pulls the family from the tanker to safety one by one, but cannot escape himself. At the last moment, as the tanker heads towards the edge of a disused bridge, Buck leaps from the tanker. As he flies over the edge of the bridge, he grabs the plastic barrier, which is quickly uprooted sending him swinging down towards the ground, as the flaming oil tanker crashes into the ground below and explodes.
Later on, while Buck was in the hospital to recover from his injuries, it was revealed that his entire adventure was actually an intelligence mission for an undisclosed intelligence agency, performed by Buck as an informal, non-official agent (thus the movie title, accidental spy). His background as an orphan, combined with his talents of extremely sharp intuitions and excellent martial arts skills had made him a perfect candidate for a freelance agent who could perform special missions. The mission was set as a "game" for Buck since he was not an official agent and therefore cannot be briefed about it. His sharp intuition enabled him to interpret his "clues" correctly, thus enabling him to perform his mission successfully.
As the end credits are rolling, Buck is shown as getting involved in another "adventure", another way of saying that he's performing another intelligence mission-as an "accidental" spy.