Secret Agent followed the exploits of John Drake (Patrick McGoohan), the hero from the previous series still called Danger Man in the United Kingdom. As before, Drake was a highly skilled agent who usually worked alone. On Secret Agent, however, Drake had become an agent for M9 (a fictional branch of Her Majesty's Secret Service). His superior was now Hobbs (Peter Madden). The cover utilized by M9 and its agents was usually that of World Travel, a travel agency which serviced a good number of destinations (some of which would definitely not welcome tourists).

Danger Man was one of the most popular TV series of its time in the United Kingdom; in fact, in the years since its original run, Britain's taste for similar such material had own grown that much greater. The Avengers, which had debuted a mere four months later, had become a phenomenon in its own right. Similar action adventure series, such as the anthology series Espionage and Man of the World (produced by Danger Man's Ralph Smart), had become rather common on British television. And putting aside all doubts that a spy craze had overtaken England were the Bond films - Dr. No and From Russia with Love had been top money makers in 1962 and 1963 respectively. The time then seemed right for the return of Danger Man.

The new series, which would debut in Great Britain in October 1964, would eventually find its way to the United States just as the original had. In America, however, the series was renamed Secret Agent in hopes of capitalizing on the growing spy craze. It was also given a new theme song, "Secret Agent Man," sang by Johnny Rivers, as well as a new tittle sequence. Secret Agent debuted in America on CBS in April 1965 and ran much longer than the original series had in the States - it even made the network's 1965 fall schedule. No doubt CBS had noticed the new series's success in Great Britain, all the while keeping an eye on the steadily increasing ratings of NBC's The Man from U.N.C.L.E.

The new Danger Man series (hereafter referred to as Secret Agent for simplicity's sake) saw Ralph Smart once again at the reins as executive producer. Initially Aida Young served as the line producer, though this function would later be filled by Sidney Cole, whose previous work included The Adventures of Robin Hood and Sword of Freedom, two midd-50's mediaeval adventure series. And, of course, Patrick McGoohan remained as superspy John Drake.

Secret Agent differed only slightly from Danger Man. Among the biggest changes was its length - the series was expanded to an hour. This resulted in episodes with more complicated plots, more action, and more character development (none of which Danger Man had ever lacked anyhow). Other changes involved the character of John Drake himself. As pointed out above, he no longer worked for NATO, but instead for the fictional agency M9. Drake's personality was also softened so as to emphasize his wry sense of humor. Secret Agent also saw Drake's sense of decency emphasized. As the new series progressed it would become more and more apparent that John Drake was a man of conscience, often unhappy with the violence which sometimes occurred in his line of work and even questioning the motives of his superiors. As i the original series, Drake never indulged in frivolous affairs with the fairer sex (not even so much as a kiss) and rarely, if ever, carried a gun.

Another change in the new series from that of the old was the increasing number of gadgets in episodes, whether due to a larger budget, their popularity in the Bond films and The Man from U.N.C.L.E., or both. During the first season such high tech gear appeared on a somewhat irregular basis; however, the second season would see gadgets increase at such a rate that at least one usually appeared in every episode. While gadgets had become more prevalent on Secret Agent, they were still a far cry from the highly advanced devices which appeared in the 007 movies or on The Man from U.N.C.L.E. Instead the gadgets were of the realistic sort that would later be seen on Mission Impossible - devices which could conceivably be built using mid Sixties technology.

I proved extremely successful, but after playing John Drake through 39 episodes of I and 47 episodes of this series, he war ready to move onto other projects - namely, The Prisoner. Following that, however, there would be two more Danger Man/Secret Agent episodes. Unlike the others, which had all been shot in black and white, "Koroshi" and "Shinda Shima" were shot in color. The two episodes aired within a week of each other, the first on June 3, 1968, and the second on June 12, 1968. These episodes would also make it to the United States, though not as part of a television series. The two episodes were edited together and released theatrically in the United States under the title Koroshi.

After its initial run, Secret Agent was released into American syndication. The 39 episodes of the half hour Danger Man were even retitled Secret Agent and given the new title sequence with the theme song "Secret Agent Man," so that they could be added to the package. Eventually, the final two episodes ("Koroshi" and "Shinda Shima") would be added to the package as well. Danger Man/Secret Agent continues to be a cult series in the United States, often considered among fans to be the prequel to The Prisoner (although whether Number Six is John Drake continues to be matter of debate).