In Biggles Follows On, Guardsman Ian Ross was a British soldier who agreed to assist Biggles by playing the role of a stool-pigeon in order to uncover the details of an intelligence operation being masterminded by Erich von Stalhein.
At the beginning of the book, Von Stalhein had been spotted in London. Scotland Yard surveillance traced him to the "Stand Easy" cafe in Caterham where he spent a lot of time chatting with troops from the nearby Guards depot, including Ross.
Biggles checks up on Ross with his superiors. Ross was an eighteen year old London born Scot. Both of his parents were British born and his father was himself a grenadier. Ross had served an apprenticeship as a motor mechanic before signing up. At the time he met Biggles, he had been in service for four months.
Questioned by Biggles, Ross revealed that von Stalhein, under the false Czech identity of Jan Stalek, was recruiting for a foreign army, a sort of international brigade, with good pay and conditions. Although Ross had no wish to desert, he said that he continued to talk to von Stalhein because he wanted to find the whereabouts of his friend Hugh Macdonald who had taken up the offer.
Biggles suggests that Ross sign on with von Stalhein. Biggles would follow him when he did. It would expose the detail of von Stalhein's operation and be the only way to rescue his friend Macdonald. Ross agrees and is taken by von Stalhein and other agents from London to Paris and then Prague.
At Prague, Ross' minder, Stresser, prevents him from communicating with Biggles, but he manages to drop Biggles a note with one word: "Kratsen". Biggles would get more details later by bribing and questioning Stresser himself. From him, Biggles learnt about the enemy propaganda base in Kratsen, Manchuria. Ross wasn't bound for Kratsen yet. He had insisted on visiting his friend Macdonald in Berlin.
Biggles followed Ross to Berlin but missed him. Ross and Macdonald had been shipped out by air to Kratsen already. Morally obliged to rescue Ross, Biggles secured government approval for a operation which was mounted for the dual purpose of rescuing Ross and destroying the propaganda base. Ross was brought home safely at the end of the story and awarded the Distinguished Conduct Medal for his part in the operation.