1935, China. The Long March hangs by a wire.

What remains of the Communist forces make a frenzied retreat to escape their Nationalist pursuers. Trudging vast distances across the nation's fractured hinterland, the only thing preventing their annihilation is the Red Army's top-secret codebreaking unit.

Led by Chief Zeng, the primitive listening posts and ox-bone abacuses of the 2nd Bureau are the sole tools at their disposal as they sift through a deluge of intercepted signals to save their comrades from enemy crosshairs. Their mission is critical when each message deciphered could save lives.

Illuminating the way to safety from the shadows, for years the names of the decoders have been lost while the great men of the revolution have been celebrated. But after all the triumphs have been committed to paper, how long can the story of their sacrifices remain hidden between the lines?