Watch An Invisible Sign (2010) HD Full Movie Free on SolarMovie. Watch An Invisible Sign (2010) online free. Mona Gray is a 20-year-old loner who turned to math for salvation as a child after her father became...Mona Gray is a 20-year-old loner who turned to math for salvation as a child after her father became ill. As an adult she now teaches the subject and helps her students through their own crises.