A professor attempts to perfect his fuel diffuser, but four planes crashes during the test flights. Peter decides to help his friend and determine if someone may be sabotaging the flights.
A woman escapes a sanitarium and come to Peter, asking his help to clear her boyfriend of a murder. When she disappears, Peter goes to see her doctor and isn't convinced by the man's claims that the woman is delusional.
Peter goes to Amsterdam to verify that a dead colleague's stolen file is authentic, and becomes mixed up with a British agent's attempts to recover the file from the spy who stole it.
Peter must cross a minefield and penetrate a military HQ to rescue a writer who has been imprisoned for speaking out against her oppressive government.
Peter's friend Stephens develops a revolutionary new anti-gravity device. An enemy agent posing as a member of a world peace organization tries to convince him to turn his secret research to the world, but does his job too well and Stephens panics and goes into hiding. Now Peter, Stephens' daughter Pat, and the enemy agent are all looking for the professor.
A USO singer inadvertently witnesses a murder at a hotel and the enemy agents responsible abduct her. However, her twin sister arrives and Peter ends up helping her track down her sibling.
Peter accompanies an attractive international weapons inspector to the Mediterranean country of Bahakim to find the source of guns being distributed to fanatics across Europe.
In France, a country doctor finds an invisible rabbit that becomes visible before her eyes, and reports it to the authorities. They in turn call in Peter to see what he knows, and the Invisible Man soon discovers a fascist plan to create an army of invisible soldiers.
A clever smuggler disguises himself as a bandaged Peter Brady to get cocaine through customs. When Peter finds out, he joins forces with Scotland Yard to smash the smuggling ring for good.
Prince Jonetta is studying at Oxford where Peter is giving a lecture. When the prince's father the king is killed, Jonetta rushes back to his homeland to be crowned, only to discover that an ambitious general killed his father when he refused to turn against the West... and the general plans to force Jonetta to either play along or die.
Enemy agents pay off a transport officer with gambling debts to them the route the Army will use to transport an experimental heat-resistant rocket. Unfortunately for them, Peter is one of the scientists involved with testing the rocket.
Captain Barry Finch is caught in a cave-in when the trench that he's planting an experimental light-sensitive mine collapses. Peter, the scientist who invented the detonator, is the only one who can get close enough to rescue Finch without his shadow setting off the mine... but sunset is fast approaching and Peter knows nothing about mine defusal.
Enemy agents smuggle an atomic bomb into England with the intent of blackmailing the government. Peter learns of the plot when a nuclear core turns up in a wrecked plane, and a second one is found in the golf bag of Lady Peversham.