Jim picks up his briefing at a library and is informed about General Casimir Zepke, who plans to make himself his country's ruler. Cardinal Souchek opposes him and has the people's support. Zepke has imprisoned Souchek and replaced him with a double who will endorse him as he takes the throne. The IMF must rescue Souchek and stop Zepke.
In the final briefing, the team tests a tube capable of projecting smoke over 40' feet. They have two days to recover the real Souchek, and every monk at the monastery where Souchek is held is a soldier. Zepke prefers to eliminate his enemies through suffocation. They go over a map of the catacombs and the door they need to get through can only be opened from the inside. Barney has mosquitoes prepared with a bacillus culture.
At the monastery, the Souchek imposter, an actor named Anton Nagorski, appears before the people on Zepke's orders, then watches the real Souchek through a one-way mirror to perfect his technique. Major Felder, disguised as a nun, calls Zepke away.
Barney and Willy sneak onto the monastery grounds and use a device to project a 40' tube up to Nagorski's window. Using an air blower, they push the infected mosquitoes into the room where they bite Nagorski. They then retract the tube and escape.
Nagorski shows a high fever and is put to bed, while Jim taps into the phone system. Zepke is not happy that Nagorski's illness interferes with his plans. He calls the hospital but the IMF intercepts the call and tells him a plague has all of their doctors occupied. Jim then punctures the tire on their car and he and Cinnamon drive up to the monastery, claiming to be a doctor and nurse. Zepke has Jim examine Nagorski, who concludes Nagorski must be kept in isolation. Zepke insists Jim give Nagorski something to revive him and Jim gives him an injection. Zepke insists that Nagoski remain at the monastery and Jim will tend to him.
Barney and Willy go into the catacombs while Nagorski starts to recover. Rollin, as a Cardinal, arrives to visit his old friend Souchek. Once Zepke is gone, Jim and Cinnamon put a compress on Nagorski's eyes and start measuring the wall of Nagorski's room, adjacent to Souchek's room. Felder visits Souchek and lets him know he'll be killed, while Rollin insists on seeing Souchek despite his "illness." While Zepke briefs Nagorski on Rollin's presence, Jim whispers the directions to Rollin based on the measurements they took. Zepke lets Rollin in to see Nagorski, and Rollin denounces him as an imposter. Zepke has him taken down to the catacombs and sealed in an airtight sarcophagus with a 300 lb. cover
Barney and Wally go into the catacombs and are sealed in by a trap, while Rollin uses the cross which contains several small metal rollers. With Barney and Willy's air running out, Rollin uses the rigged cross as a jack to lift the lid enough to slip the metal rollers underneath it. Once the guards leave, Rollin slides the lid aside on the rollers then opens the door to the catacombs without activating the deadfall.
Jim uses a small hidden device to heat up Nagorski so he has a relapse, then brings in an oxygen tent. Barney, Willy, and Rollin slip into Souchek's chamber. They stay clear of the one-way mirror, silently brief Souchek to maintain a normal presence, and Rollin disguises himself as Souchek/Nagorski. Rollin then takes Souchek's place while Barney and Willy burn through the indicated section of the wall behind Nagorski's bed.
Jim sets up the oxygen tent and aerates it with water vapor so that Zepke can't see inside. They then slip out the unconscious Nagorski but Zepke hears something and orders them to open the oxygen tent. Cinnamon distracts him by pointing out he has a lit cigarette and the team gets Souchek in to take Nagorski's place in the oxygen tent and makes a rapid recovery. Once Rollin-as-Souchek has convinced Zepke he's still imprisoned, he removes his Souchek disguise and slips out with Barney and Willy disguised as reporters where they blend into the press.
Zepke orders Felder to dispose of "Souchek" (Nagorski) then goes outside to tell the press he and Soucheck have reconciled. Souchek comes outside… and denounces Zepke. Souchek leaves as the team make their escape separately and Felder informs Zepke what has actually happened.