TELEVISION / And how they face their final curtain: Morse is . . . well, that would be giving it away. Mark Lawson on honourable retirement and happy endings

The last acts of vast television successes possess an emotional charge of which there can be no exact equivalent in other narrative arts. Actors and crews have spent years of their lives together, viewers extended their own brand of collaboration over the same period. (Even at the end of long stage runs, it is improbable that 75 per cent of the audience would have seen every previous show.)

Filming Hilda Ogden's tearful farewell to Coronation Street, it is doubtful that Jean Alexander and her colleagues had to do much acting, for a working relationship of more than 20 years was simultaneously ending. M*A*S*H was always a weepy series, but, in the two-hour closing show, the entire cast seemed to be wearing sequinned contact lens. Tonight's final exchange between Morse (John Thaw) and Lewis (Kevin Whately) in Inspector Morse has such a heavy tenderness - for what is, logically, just another office goodnight - that it leads you to believe that the Chief Inspector must surely be shot as he walks to his Jaguar. But, whether or not he is, the moment is the death of a character, the last shot.

How to finish a long-running series is the last treat for producers and writers. The valedictory Dallas sent down Joel Grey as the Recording Angel to play highlights of all the previous series to a repenting JR. The Blackadder comedies, with absolute finality, sent the whole cast over the top in the Great War. Inspector Morse - in 'Twilight Of The Gods', Julian Mitchell's script for the 28th two-hour story - opts for a clever compilation of running gags and ceremonial formalities.

There is a teasing moment when Morse seems about to disclose his deep-sixed Christian name in a flower shop and the show's resolute bookishness, unusual for a popular TV series, culminates in a friendly nod at James Joyce - the entire action, two murders and their solution, is played out on one summer day - and a fatal shot is fired from the Bodleian. And the Oxford-based series tackles the city's most recent piece of history in the character of a foul-mouthed, helicoptering tycoon, who uses the British libel laws to whitewash his horrors.

And so ends one of British television's most curious howdunnits: the trick of making literate, visually plush drama for an audience previously available only to formula soaps (18 million in Britain, 750 million worldwide). In unravelling the mystery of its popularity, the key evidence is two decisions taken by Kenny McBain, the series' first producer, who died young in 1989. The first was the problem of how faithful to be to the source novels, by Colin Dexter, which McBain had bought up. In opting for an almost Royal level of infidelity, McBain has created a significant example of the box bettering the book.

On the page, there is something unpleasantly creepy about the emphasis on Morse's autumnal sexual desires, and Sgt Lewis is an elderly Welshman. (Recent Dexter books have moved into line with television.) McBain's other winning calculation was of format, choosing two-hour, free-standing episodes over the familiar one-hour or half-hour serial style. It is worth remembering that, when Morse began, the emergent media idea was that of the three-minute culture, a short-attention audience, which would force all drama to the condition of rock videos or commercials. McBain and Central Television more optimistically gambled on creating television with a cinematic sense of event, length and lushness.

The series' greatest achievement, though, was to bridge the brows of television drama. The best Morses can be decoded at both high and low levels. These two-tone texts were pioneered by Julian Mitchell, a stage playwright (Another Country) who has written nearly a third of the Morse scripts. His 1990 episode Masonic Mysteries, one of my personal top two, could be viewed as a conventional murder teleplay - in which the detective falls under suspicion - but was also a detailed parody of The Magic Flute, via an amateur production in which Morse was appearing, complete with satirical sideswipes at British police freemasonry and plot and soundtrack quotations from Schikaneder and Mozart. (Barrington Pheloung's original scores for Morse have also generally contained bonus jokes for the musically literate viewer.)

Inspector Morse can, I know, be accused of nostalgic softness. Its vintage cars and beer-swilling coppers can be seen as Fifties flashbacks: an exhumation of an innocent England, now ruined, with Morse as a sort of slightly updated, male Marple. I refuse to accept this prosecution. It is true that, when compared to a realistically contemporary police series like BBC1's recent Between The Lines, Inspector Morse looks tactfully propagandist about both country and cops. But, if Morse is to be called escapist nostalgia, what words would be left for The Darling Buds Of May? In fact, Morse is daringly downbeat for a mainstream series. The improbable body-count for a rural and university beat - many of those 750 million foreign viewers must think Oxford is England's Detroit - is in one way a concession to genre but, as the contemporary experience of most viewers is of violence spreading ever closer to previously reliable tranquilities, may also serve as a microcosm of modern fears.

Julian Mitchell's Another Country was about national hypocrisy, betrayal, decline and the British class system. His Morse scripts have consistently smuggled these themes into popular culture. Broadcast during a period when real-life belief in the police was retreating weekly, the show admitted a psychopathic Chief Inspector in Second Time Around (1991) and, in Mitchell's Promised Land the same year, my other favourite, made Morse the first mainstream TV detective to be burdened with a wrongful conviction. (Second Time Around was screened 13 days after the release of the Birmingham Six.)

There hangs around Morse a terrible malaise - an exhaustion and gradual loss of certainties (the one about Art goes on the bonfire tonight) - which nicely suits the present national mood. This atmosphere is immensely abetted by John Thaw's tortured performance, the best bit of full-frontal facial acting since Alec Guinness in the Le Carre dramas.

Morse's first words in his last television appearance are 'More] More]', ecstatically yelled at the great Welsh soprano Gladwys Probert, at the end of her master-class in Oxford. Morse viewers may also mournfully roar for an encore at the conclusion of tonight's mystery. However, Morse, as an opera fan, will be well aware of those careers which have gone on too long, straining for the high notes in front of an audience willing itself still to see what once it saw.

The last two years have offered too many interchangeable cat-and-Morse plots. The first episodes of this final series seemed, for the first time, operatic in the pejorative sense. The opening show at one point had a character in a coma, a second in intensive care and a third dead, like some visual text-book on terminal medicine. Even in tonight's Mitchell script, the egg-head and pea-brain contrast between Morse and Lewis is archly pushed too far. Interrupted on the brink of setting a personal crossword record, Morse moans: 'You, Lewis, are the person from Porlock.' Lewis chirps: 'No, sir. Newcastle.'

Showing better judgment than many of his hero tenors, Morse, the great lyric copper of his generation, has known when to take the final bow. But, before he goes, let us admit these names to television's hall of fame: the late Kenny McBain, John Thaw, Kevin Whately, Julian Mitchell, Barrington Pheloung.