"River of Missing Men" is the story-source title and, briefly, the working title of this film while in production, but it is not an "alternate" title since "Trapped By G-Men" was never shown anywhere under this "supposed and incorrectly-tagged" alternate title. This film, which also served as the plot basis and provided some stock footage for Jack Holt's 1941 Columbia serial, "Holt of the Secret Service" opens with Harry Grady and Fred Drake, the latter suffering from a bullet wound after making a getaway from a bank robbery, discovered hiding out in an auto camp near Tucson. Drake is arrested but Grady escapes. In Washington, D.C., FBI Chief Conover calls in his best operative, Martin Galloway, and assigns him to the case. Galloway assumes the identity of bank robber Bill Donovan, believed to have drowned in a river without disclosing where he had hidden $200,000 in stolen money. Galloway, nee Donovan is "captured" and put in the same infirmary with Drake, planning to escape and let Drake lead him to Grady's sanctuary. Agent Alice Segar is posing as "Mrs. Donovan" and, with the warden's aid, assists Galloway and Drake in their escape. Drake forces Alice out of the escape car, but leads Donovan to the banks of a river, where they take a canoe down river to the gangs hideout at a deserted mine between some bluffs. Hidden there are fugitives Grady, Lefty, Kilgour, Blackie and several other highly-wanted fugitives, and they hold captive mine operator Dad Higbee and his daughter Nancy. Lefty, who has gone to town for supplies, is almost captured by Alive and other G-Men but escapes by forcing a young tourist, Dick Withers, to man a canoe and paddle him back to the hide-out. Galloway/Donovan takes Nancy and Dick into his confidence, while Blackie, sent to tell "Mrs. Donovan" to bring the $200,000 in loot to the hide-out as proof of "Donovan's" sincerity, is followed back to the hide-out by Alice and the G-Men.