The professor's invention of impervious light metal is significant to the war industry and the kidnappers' motive is not financial, but political.

Nicke, a kidnapper, says that he "could do what else for the thing and everything was right which could promote the thing" ("kunde göra vad som helst för saken och allt var rätt som kunde främja saken"), but the engineer Peters threatened him: "Jag behöver väl inte påminna dej om hur det går för såna som försöker hoppa av?"

One day when Kalle, Eva-Lotta and Anders pass, they meet a 5-year-old boy called Rasmus who lives there with his father professor Rasmusson, who tells that he has invented an impervious light metal.

One night after the "war" at the castle ruins, Kalle, Eva-Lotta and Anders see three men in a car who kidnap Rasmus and the professor and when Eva-Lotta tries to rescue Rasmus, the kidnappers force her to go with them in their car, but she throws out cakes and papers along the way so Kalle and Anders, who go after them by the professor's motor cycle, can find them.

They steal a boat and row over and go quickly by the motor cycle to Eklund's house and find the documents when the kidnappers come.

Nicke tells that at tomorrow evening a floatplane will come and take Rasmus and his father to another country.

The next morning when they are leaving the island, the kidnappers find them, tie them and take them back to the house and lock them in.

When it's dark, Kalle sneaks around among the houses and hears Blom saying that the floatplane will come tomorrow morning 7 o'clock and understands that they have a two-way radio.

The pilot tries to drive upwards but can't and due to the damaged float the plane tips over and sinks in the water.