Mr. Ray spent several weeks in Toronto immediately after the assassination. Then he flew to London, to Portugal and then back to London, where he was captured by Scotland Yard on June 8 as he was about to board a flight to Belgium. In his possession were two Canadian passports and a loaded gun.

Back in Memphis on March 10, 1969, his 41st birthday, Mr. Ray stood before Judge W. Preston Battle of Shelby County Criminal Court, pleaded guilty to the murder of Dr. King and was sentenced to 99 years' imprisonment, with no eligibility for parole before serving half that term. The plea, which waived his right to trial, enabled him to escape the electric chair.

At one point in that hearing, both the prosecutor, P. M. Canale, and Mr. Ray's lawyer, Percy Foreman, told the court that there was no evidence that anyone else had conspired with Mr. Ray to kill Dr. King. But the defendant then leaped to his feet and declared that he did not intend that his guilty plea be construed as meaning that there had been no conspiracy.

In the quarter-century since, the idea that Dr. King was a victim of conspirators has refused to die. Indeed, in 1978, at the end of its two-year inquiry that also looked into the slaying of President John F. Kennedy, the House Select Committee on Assassinations concluded that although it was Mr. Ray who had fired the shot that killed Dr. King, circumstantial evidence pointed to the ''likelihood'' of a conspiracy. (The committee also concluded that President Kennedy ''was probably assassinated as a result of a conspiracy.'')

In any event, only three days after entering his guilty plea Mr. Ray wrote the judge that he wanted to withdraw it. Within a week he sought a trial, an effort that would last till he died. Testifying in 1974 in furtherance of that effort, he maintained that his involvement in the assassination had been only peripheral but that Mr. Foreman, his lawyer, had seemed intent on persuading him to plead guilty. He went along, he said, because he believed that otherwise Mr. Foreman might ''fake'' a defense at trial and that he might then be executed.

But if his own role was peripheral, whose was central? Mr. Ray never told all that he said he knew.

''If I inform and get out,'' he once said to an interviewer in prison, ''they will solve their case. I'll go back to the Missouri penitentiary where I owe them 15 years, and I'll get murdered as an informer. So if I inform it will work out for them, but it won't work out so well for me.''

Mr. Ray was a fugitive from that Missouri prison at the time of the assassination, having escaped on April 23, 1967, while serving a sentence for a holdup. In the year between that escape and Dr. King's murder, conspiracy theorists wondered, where did he find the money to live on?