Of all the international pop-culture icons who met an untimely death—like Marilyn Monroe, Elvis Presley or Michael Jackson—Bruce Lee is perhaps the only one for whom there is no official consensus as to the cause. For 45 years, fans, experts and forensic pathologists have offered different theories, ranging from the supernatural (killed by an ancient curse or bad feng shui) to the ridiculous (poisoned by Japanese ninjas) to the ironic (the figure known for the catch phrase "Be like water" drank too much water). Conspiracy theorists blamed his luckless mistress, painting her as a sinister black widow.

Now, the first authoritative biography of the crossover martial arts movie star, Bruce Lee: A Life, reveals the true timeline of his last day and a compelling new explanation for his demise.

His final day on earth started well. On the morning of July 20, 1973 in Hong Kong, the 32-year-old box-office phenom met with Australian actor George Lazenby, who played James Bond in On Her Majesty’s Secret Service (1969), to offer him a part in his upcoming film Game of Death. Lee, a child actor who went on to international fame for hits like Fist of Fury and the TV show “Green Hornet”—and was arguably responsible for popularizing the martial arts film genre in the West—had already begun producing movies himself.

After meeting with Lazenby, Lee decided to visit the apartment of his mistress, Betty Ting Pei, for a “nooner.” Around 6 p.m., Raymond Chow, Lee’s business partner, arrived. The three of them were scheduled to meet Lazenby for a celebratory dinner, but before they left, Lee complained of a headache. Betty gave him one of her prescription pain medications, which contained aspirin. Lee told Chow to go on without them. He went to lie down on Betty’s bed—and never got back up again.

When Betty couldn’t wake Lee two hours later, she called Chow at the restaurant in a panic. He raced over to her apartment, but it was too late: The most famous man in Hong Kong was already dead. To avoid a scandal, Chow called an ambulance and had Lee transported to a nearby hospital, where the doctors continued to work on his lifeless body before declaring his time of death as 11:30 p.m. Chow then told the media that Lee had collapsed at home with his wife, Linda.

But when an intrepid reporter uncovered the truth three days later, the tabloids went wild. The China Star ran a double-entendre headline: “Betty Ting Pei’s Fragrant Chamber Killed the Dragon.” Andre Morgan, who worked with Lee and Chow, says, “The stories were rampant: stories about him dying from an overdose, dying from screwing too much, dying with an erection, dying from being hacked to death by young thugs, poisoned by his servant. There was one story that he wasn’t really dead.”

This fevered speculation had real-world consequences and quickly took a turn for the truly frightening. Students in Kuala Lumpur demonstrated, carrying placards that read: “Betty Killed Bruce.” Rumors began spreading in Hong Kong that a hit had been taken out on her life. In early August, a bomb threat was called into the police, who discovered in a public square a suspicious brown paper package covered in Chinese writing: “Betty Ting knows the cause of Bruce Lee’s death.” The bomb turned out to be a hoax, filled only with rubbish, but over the next few weeks three more fake bombs were planted across the city with such messages as “Revenge for Bruce Lee.”

The British colonial government could safely ignore a celebrity scandal, but bomb threats were another matter. To restore confidence and calm, officials ordered a full-scale inquest into Bruce Lee’s death. The problem was, none of the experts could agree on why Lee died. The autopsy had revealed the medical reason—cerebral edema (swelling of the brain)—but the coroner had no idea what had caused it. Two of Bruce’s doctors blamed the hash he had consumed that afternoon, but the idea was quickly dismissed since it is scientifically impossible for cannabis to cause cerebral edema. With the investigation at a standstill, the government flew in an expert from London who offered a novel hypothesis: severe allergic reaction to aspirin, or anaphylactic shock.

Without any better options, the government accepted this conclusion and tried to move on.

Most of Lee’s fans did not. Lee was a hardcore martial artist who had taken aspirin most of his adult life without any side effects. Moreover, anaphylaxis, a severe allergic reaction, is almost always accompanied by other symptoms—an enflamed trachea, neck, tongue and lips, as well as hives and red itchy skin in and around the mouth. In fatal cases, the swelling of the throat blocks the airway resulting in asphyxia and cerebral edema. The autopsy revealed no symptoms of anaphylaxis. It couldn’t have been an aspirin allergy that killed Bruce.

The experts were so focused on what had happened on July 20 that they failed to adequately consider earlier evidence. Several months before his death, Lee had an operation to remove the sweat glands from his armpits, because he thought dank pits looked bad on-screen. This reduced his body’s ability to dissipate heat. Ten weeks before his death on May 10, Lee walked into a tiny dubbing room to re-record dialogue for Enter the Dragon. The engineers turned off the air conditioner to avoid having its noise ruin the soundtrack. After about 30 minutes in this sauna-like room, Lee fainted and started convulsing. He was rushed to the hospital and nearly died from cerebral edema. The doctors diagnosed and treated it in the nick of time.

None of them realized his collapse was most likely due to heat stroke, one of the most common killers of young athletic men in the summer months. In the United States alone, an average of three high school and college football players die every year of heat stroke. A common finding in the autopsy of heat-stroke victims is cerebral edema. “A person who has suffered one heat stroke is at increased risk for another,” says Dr. Lisa Leon, an expert in hyperthermia at the U.S. Army Research Institute of Environmental Medicine. “Patients experience multi-organ dysfunction during the hours, days and weeks of recovery, which increases the risk of long-term disability and death.”

According to records at the Hong Kong Observatory, July 20, 1973, was the hottest day of the month that year in tropical Hong Kong. The oppressive heat was weighing heavily on Lee and Chow. “Bruce wasn’t feeling very well,” Chow told me, revealing details never previously reported. “I wasn’t feeling very well either. I think we had some water, and then he was acting.” In Bruce’s bubbling enthusiasm over Game of Death and Lazenby’s potential participation in it, he jumped up and began performing scene after scene. “He was always very active,” Chow told me. “In telling the story, he acted out the whole thing. So, that probably made him feel a little tired and thirsty. After a few sips, he seemed to be a little dizzy.”

At this point, he complained of a headache, a common symptom of hyperthermia, took the pain medication Betty offered him and went to lie down. Unlike on May 10, no one suspected anything was wrong and he died before anyone could get him to a hospital to treat him for the cerebral edema—which is retrospect seems clearly to have been caused by heat stroke.