Two American lads backpacking across Europe get caught out on moors in the north of England on the night of a full moon. They are attacked by a wild animal. One is killed, the other traumatised by his injuries. He is even more traumatised, however, when his dead friend shows up to tell him that on the next full moon he will turn into a wolf and start eating people, so he should kill himself immediately.

Hard though it may be for some people to hear, AN AMERICAN WEREWOLF IN LONDON turns 40 this year and in all that time, it has never been surpassed as the benchmark against which all other werewolf films are measured and found wanting.

Comedy horror is a notoriously hard genre to pull off. The comedy undermines the horror and it's hard to laugh at someone's innards being spilled across the screen. AN AMERICAN WEREWOLF IN LONDON is a masterclass on how to combine the two in a way that complements both and raising the whole above the sum of its parts. As writer and director, John Landis is the only man who can take the credit for that, making sure that both get equal amounts of screen time and both are equally effective. His deftness of touch with both elements ensures that the film satisfies on all levels, playing with the formula whilst adhering rigidly to it. For example, the scene in which the fearful locals warn the boys not to up to Dracula's castle - sorry, out onto the moors- is by the book Hammer horror. Its very familiarity makes it funny, as do the reactions of the uncouth Americans, but it is played straight by the British cast and directed just off-kilter enough to be unnerving at the same time.

There then follow sequences of straight horror (the pursuit of a commuter in the strangely empty Underground station, the dream sequence) and straight comedy (the victims' group meeting in a sleazy sex cinema is hilarious, waking up naked with the wolves in the zoo) and both are just as memorable for being judged to perfection.

Even so, it's a werewolf story (no spoiler, it's right in the title) and any werewolf story lives, dies or becomes undead through the quality of its werewolf, especially the transformation from human to wolf. Rick Baker's in-camera effects work on the transformation in a London flat is astonishing, looking as good today as it did forty years ago. There have been many werewolves since, but none has had a transformation sequence as good as this one. It puts all the embarrassing CGI efforts in films like the UNDERWORLD series to shame. There isn't a moment when you don't believe this is really happening and it looks (and sounds) really, really painful. This one sequence is enough on its own to keep the film its 'best werewolf' title. Nothing has ever come close.

With werewolves and monster nazis and the walking dead to deal with, the cast is vital to ground the story in reality and here Landis strikes pure gold. David Naughton is a very likeable, unassuming lead, but he is playing straight man to Griffin Dunne's Jack, who initially appears to have a limited part in the film, but then comes back in states of ever-increasing decay, making jokes and telling his friend to kill himself. The contrast of the funny and macabre in these sequences works because of Dunne's natural comic timing and Naughton's believability as he struggles to believe any of what is happening. You can believe these two as lifelong friends and, through them, believe the events of the film. Landis then adds to that the presence of Jenny Agutter as the sexy girl next door no-nonsense nurse who takes David in. She again plays it absolutely straight, as though she is in an episode of Downton Abbey, not some horror flick. She is the perfect English rose and never more sexy. She creates the film's heart and the burgeoning love story never feels forced or false. Completing the main quartet is John Woodvine as a doctor who begins to suspect the truth. His controlled performance is exactly what the role needs.

The bloody effects are squelchy enough for horror afficianados, especially the corpses of the werewolves' kills, and the climactic mayhem in Picadilly Circus is a parade of unexpected, violent death. If there is any flaw with the film (and no film is perfect, after all) it is the abrupt ending, which reaches a conclusion before just... stopping. As a complaint, it's a pretty minor one.

AN AMERICAN WEREWOLF IN LONDON is every bit as enjoyable today as it was when it first saw the light of a projector screen forty years ago and it's going to take something very special indeed to take it's best werewolf film ever title.