Ask any fan of 1960s spy show ‘The Avengers’ and they’ll all tell you the same thing – series four was the point where it started to go seriously bonkers.

Chances are those same people will also tell you that they are arguably the best set of twenty-six episodes offered up in its entire eight year run.

By late 1964, as ‘The Avengers’ was gearing up for its third full series, Honour Blackman had suddenly announced that she would be quitting the show after filming was complete. The actress had been offered a five-year film contract, the first of which would be starring as Pussy Galore in the next James Bond film, ‘Goldfinger’.

Actress Elizabeth Shepherd had initially been cast in the role of new sidekick Emma Peel, even getting as far as filming one and a half episodes, before it became clear she wasn’t suitable for the role. Production had shut down for several weeks while the programme-makers desperately cast about for another actress to fill the role, eventually settling upon 28year old Diana Rigg.

For many fans Rigg’s Mrs Peel is considered to be the quintessential sidekick, and her three years on the show is often cited as the most memorable period in the programme’s history. Watching ‘The Avengers: The Complete Series Four’ DVD box set, it’s not hard to see why.

Many of the episodes found spread across this seven-DVD set are vastly different, in both style and content, to episodes that were transmitted only a year previous. In an attempt to sell the show to a US audience the series has undergone a radical reformatting, visually having more in common with Brian Clemens’ later offerings, such as ‘The New Avengers’ and ‘The Professionals’.

Now shot entirely on film and making extensive use of location filming, the series has a glossy, almost American feel to it. Far removed from the gritty little homespun espionage thriller of its first three seasons, the series now sees its central characters tackling the dastardly plots of criminal masterminds and crackpot scientists.

With classic episodes such as ‘The Cybernauts’ and ‘Man-Eater of Surrey Green’ the series is beginning to lean further towards science fiction, with Steed and Mrs Peel battling against killer robots and carnivorous extra-terrestrial plant life respectively.

It’s not hard to see why this series left such an indelible mark on the British viewing public some forty-five years ago, an audience more accustomed to kitchen-sink dramas and ‘The Wednesday Play’.

In Clemens’ hands the series is steadily drifting further away from creator Sydney Newman’s original brief and moving inexorably into a territory known as Spy-Fi; something that had already become increasingly popular through James Bond movies and ‘The Man From U.N.C.L.E.’ Now, behind every shady politician there is a mad scientist hell bent on global domination, behind every political assassins there is a chess-obsessed billionaire looking to expand his hotel empire.

But, more importantly, series 4 presents us with a show that has broadened its scope, offering up a wider variety of tales that touch upon many easily identifiable genre tropes. From post-apocalyptic paranoia to alien invasion, from brain-washing to global terrorism, ‘The Avengers’ is constantly reinventing itself to meet a rapidly developing media market place. Further evidence of which is clearly seen the following year, with its move into colour, purely with the intention of selling the show to the U.S networks.

Once again Optimum and Studio Canal have managed to pull together a vast array of extras for a show that celebrates its 50th anniversary this year. With commentaries from writers, producers and directors, episodes of ‘Armchair Theatre’, series 1 episode reconstructions and archive news footage this is quite possibly the most impressive collection to date.

Yet, its Optimum’s attention to detail that really sorts out the geek from the chaff. It’s the addition of the famous USA chessboard opening sequence, or ‘The Strange Case of the Missing Corpse’ mini episode and ITN newsreel footage that really make these collections stand out from, say, the region 1 megasets.

For all those casual viewers out there who’d like to get hold of a few episodes just to reacquaint themselves, or perhaps to see what all the fuss is about, then ‘The Avengers: Complete Series 4’ box set is the perfect jumping on point. Perhaps its only sin is that, on occasion, it has the tendency to be strange and surreal just for the sake of it – Sir Horace Winslip, the steam railway obsessive who’s house interior is designed to resemble a train carriage, signal box and rail track, being a perfect case in point.

In the end series 4 breathes much needed new life into a television series that was in danger of out staying its welcome. A solid set of twenty-six episodes, with barely a below par entry amongst them.