1968's Il mercenario (titled The Mercenary during its North American run, but since then known as A Professional Gun in the English-speaking world) was one of the most commercially successful examples of the form, but it wasn't particularly well respected among the more serious minds in Zapata western circles, for reasons we shall attend to presently. The film did, however, reunite director Sergio Corbucci and star Franco Nero, the team behind 1966's smash hit Django.
Sergei Kowalski (Nero, barely recognisable from his star-making role thanks to some magnificent facial hair) is a Polish gunslinger who is hired by Mexican mining barons to transport a load of silver to the United States, protecting it from the rebels roaming the area. Arriving at the mine, Kowalski finds the place in the hands of revolutionaries led by Paco Román (Tony Musante).
When the army attacks, Kowalski lets Paco pay him handsomely to fight them off with the machine gun he carries around ('Two hundred [dollars], you bastard - here, I hope you spend it on doctors!', Paco curses). The next day, Kowalski is accosted by Curly (Jack Palance), a villainous American trying to get his hands on the silver, but he is rescued by Paco's men. Curly swears revenge for the death of his men (one of whom, it is implied, was his lover), but is stripped naked and forced to walk back to civilisation by the rebels. Thereafter, Paco hires Kowalski as military advisor for the revolutionary war.
At a purely narrative level the film feels rote and uninspired. What isn't boilerplate is awkwardly shoved in: the character of Columba (Giovanna Ralli), for example, serves little purpose except as plot device and eye candy. And the film suffers from a series of false endings, stopping and starting repeatedly in its last fifteen minutes before coming to a close with a message that doesn't really respond to the arc of the character it's imparted to.
Nor would it be fair to accuse Corbucci of frivolity, and that makes it even stranger: that same year, after all, saw the release of The Great Silence, a film so grim and nihilistic it pushed the spaghetti western into and out the other side of parody. In other words, Corbucci's first entry into an ordinarily serious subgenre was pretty silly - no film in which one of the parties enters the climactic gunfight wearing a clown costume can claim to be all that straight-faced - while the 'standard' spaghetti western, which left more leeway for and would eventually descend into lightheartedness, was where he chose to place the darkest of his works.
Even though it does not deliver on the promise of its beginning - or, judging from the sort of person Corbucci was, possibly deliberately baits the audience - Il mercenario is far from a waste. Nero's self-amused bastard is as fantastic as ever, and he absolutely rocks the scenes in which he grants himself special treatment just to anger Paco (a device that would be reused by Eastwood in High Plains Drifter). Corbucci's style is nothing to sniff at, either: besides signature moves, the showiest bit of direction is a 360-degree tracking shot of Curly riding in a circle while a man is beaten offscreen. It's not great Corbucci, but is it ever interesting.