During a crippling Tokyo heatwave, a homicide detective (Toshiro Mifune) has his gun stolen out of his holster while taking the tram. When the stolen gun is used to murder someone, the desperate rookie teams up with a veteran detective (Takashi Shimura) to track the killer down.

Watch Akira Kurosawa’s films in chronological order, and you can see his directorial and storytelling styles develop and gradually mature. There are some excellent films among his first eight works: Sanshiro Sugata, The Men Who Tread in the Tiger’s Tail and One Wonderful Sunday. His ninth film, the 1949 police thriller Stray Dog, is Kurosawa’s first bona-fide masterpiece. It is his third collaboration in a row with actors Toshiro Mifune and Takashi Shimura, and the working relationship pays remarkable dividends. This is not simply an outstanding movie: it is one of the most significant motion pictures ever made.

The most obvious derivative work at which to point is David Fincher’s 1995 thriller Seven, which shares so many similarities that it is obvious one film is heavily indebted to the other. It doesn’t stop at Seven, however: Stray Dog is the first-ever film to match two cops – one a cynical veteran, the other an upstart rookie – and send them off to fight crime. If you’ve ever watched a buddy cop movie in your life, you basically have Akira Kurosawa to thank for it.

By his own admission Kurosawa wasn’t aiming to invent anything: Stray Dog was developed as a Georges Simenon pastiche, but mutated through the scripting process into its own highly distinctive form. It is a hot summer in Tokyo, and Kurosawa really makes the audience feel it. The actors shine with sweat, and constantly mop their brows with handkerchiefs or greedily drink in bars. In one wonderful moment Shimura’s Detective Sato irritably grabs an electric fan from a witness and forces it to turn back and forth between them. The heat gives the film a sense of overwhelming oppression, and an atmosphere far and away beyond anything Kurosawa has previously done. You wind up waiting for the rains to come, which they do – deliberately timed to enhance the film’s climax.

The film’s narrative follows a procedural course, concerned less with action-packed chases and gunfights and more with dedicated and smart police work. Sato and Murakami (Mifune) follow a lead, speak to a witness or arrest a suspect, and use information or evidence to take them to the next lead. The film’s villain, a former soldier turned violent robber named Yusa (Isao Kimura), doesn’t even show up until very late in the film – one of the many narrative tricks that Andrew Kevin Walker lifted for his Seven screenplay.

In between their investigations Sato and Murakami debate the nature of criminals: are they victims of circumstance, as Murakami believes, or simply bad people, as Sato insists? There’s an ambiguity to this films treatment of criminals and crime that really makes Stray Dog stand out. It’s post-war setting enhances this, or perhaps directly inspires it: there was widespread poverty in Japan in the late 1940s as well as extensive rationing. Rice ration cards form a key piece of evidence in the plot. An early and deliberately lengthy sequence sees Murakami go undercover as a down-on-his-luck drifter, hoping to meet a illegal gun trader while on the street. He’s largely unsuccessful, because there are simply too many people out on the streets for him to make contact with anybody. The location scenes of this prolonged montage were shot by Kurosawa’s assistant director Ishiro Honda, who would go on to direct films for Toho himself – including Godzilla and The Mysterians.

Technically the film sees Kurosawa employ his entire bag of cinematic tricks: rapid Hollywood-style cutting, montages, complex tracking shots, low and high angles, and so on. It’s by far the most visually interesting film he has directed to this point.

Kurosawa himself was not an enormous fan of Stray Dog, finding it technically strong but emotionally cold. He was under-selling his own work. At the film’s centre there is a remarkably emotional performance by Toshiro Mifune. When his gun is stolen he panics. When it begins to be used in violent robberies and murders he despairs. When he learns the criminal he is chasing shares so many similarities with him he sympathises. It’s a great performance of a great character in a great film.