Berlin, 1967.
Three weeks after moving to West Berlin for his diplomat father's work, American Harry Rogers witnesses the shooting of a boy trying to escape into the West over the Wall.
In East Berlin, Jakob is the adopted son of a high up Stasi officer, Hans Eberhardt.
When Harry floats a helium filled balloon with a message attached, Jakob chances upon it and sees an opportunity. The boys begin a correspondence. On the surface their letters appear to be those of two normal pen friends, but beneath these they use lemon juice as invisible ink to share messages that would not make it past the authorities. They soon realise that a hidden tunnel is their only way out as Harry and Jakob race time and test their wits against the watchful eyes of the Stasi.