Henry Plummer left Gold Creek on September 21, 1862, taking the Mullan Road over the Continental Divide then down the Missouri to Fort Benton, a distance of about 200 miles. He almost certainly arrived at Fort Benton before October 1.

You might want to look at the article on Mountain Steamboats

Francis Thompson had come up the Missouri on the steamboat Emilie to Fort Benton in June of 1862. Francis Thompson was nominally a goldseeker but became a merchant instead--a surer way of getting rich at the mines. James and Martha Vail were on the same voyage, along with Martha's younger sister Electa Bryan. The Vails were on their way to the government demonstration farm at Sun River, established to try to get the Blackfeet to take up farming. James Vail had been hired by the Dakota Territory Indian Agent to spend a year at the farm.

Thompson kept a journal, which has never been made public. Instead, he published his "reminiscences" in 1913, 50 years after the events of 1862-1864. It is from these we know that Plummer and Jack Cleveland, an acquaintance from California, were in Fort Benton looking for passage down the Missouri in a mackinaw boat. The danger from Indian attacks along the Missouri that fall was so great that they could not find anyone to take them down.

Meanwhile, back at the government farm, with only the Vails, Electa, young Joseph Swift and one Indian hunter, the Indian danger had become so great that James rode over to Fort Benton to seek help. He engaged Plummer and Cleveland to return with him to help guard the farm. We cannot be sure of the exact date, since there is no known journal or diary from the Vails. It must have been around the first of October. Joseph Swift kept a journal but it has never surfaced.