The Hidden Code (1920)

Quirky chemist Eben Lamont has discovered a powerful new explosive and plans to turn it over to the government. Richard Leslie, the head of an explosives manufacturing company, offers to buy his new discovery, but Lamont turns him down. Afraid that his formula will be stolen by Leslie, Lamont uses invisible ink to tattoo part of the formula on his daughter Grace's shoulder. A spy for a foreign government finds out about it, kills Lamont and kidnaps Grace, hypnotizing her so she'll tell him where the formula is. She is rescued by Leslie in the nick of time, but her troubles, as it turns out, are not yet over.