The Rumpo Kid rides into Stodge City and terroises the community, aided by his band of roughnecks and layabouts, until Marshall P Knutt, a keen and conscientious sanitry engineer, is mistaken for a law marshall. With a helping hand from revenge seeking Annie Oakley, he proves in the final shoot-out that there is more to drains than dirty water.
Outlaw Johnny Finger, better known as The Rumpo Kid (Sid James), rides into the frontier town of Stodge City, and immediately guns down three complete strangers, orders alcohol at the saloon—horrifying Judge Burke, the teetotal Mayor of Stodge City—and kills the town's sheriff, Albert Earp. Rumpo then takes over the saloon, courting its former owner, the sharp-shooting Belle, and turns the town into a base for thieves and cattle-rustlers.
In Washington DC, Englishman Marshal P. Knutt, a "sanitation engineer first class", arrives in America in the hope of revolutionising the American sewerage system. He accidentally walks into the office of the Commissioner, thinking it to be the Public Works Department, and is mistaken for a US Peace Marshal, and is promptly sent out to Stodge City.
The Rumpo Kid hears of the new Marshal, and tries all he can to kill the Marshal without being caught, including sending out a pack of Indians, led by their Chief Big Heap and hanging the Marshal after framing him for cattle rustling. Knutt is saved by the prowess of Annie Oakley, who has arrived in Stodge to avenge Earp's death and has taken a liking to Knutt.
Eventually, Knutt runs Rumpo out of town, but once Rumpo discovers that Knutt really is a sanitary engineer and not the Peace Marshal he once thought, he swears revenge, returning to Stodge City for a showdown at high noon. By hiding in the sewers beneath the main street, Knutt kills off Rumpo's gang, but fails to capture Rumpo, who escapes with the aid of Belle.