Scientist astronautHarrison "Jack" Schmitt, a geologist, persuades his mentor, professor Lee Silver, to train the Apollo astronauts in selecting appropriate rock samples to collect through field experience, rather than the boring classroom lectures NASA has been using. Silver takes the four Apollo 15 prime and backup landing crew members (David Scott, James Irwin, Richard F. Gordon, Jr., and Schmitt) to the southwestern desert, while lunar geologist Farouk El-Baz trains the Command Module pilots (Alfred Worden and Vance D. Brand) in high-altitude recognition of geological features using airplane flights over Hawaii. Schmitt is disappointed to learn his own Apollo 18 flight will be cancelled, but he still believes the training of the other astronauts is vital. It pays off when Scott and Irwin find the "Genesis Rock", originally believed to come from the Moon's primordial crust. The title refers to Scott's reproduction of an experiment proving Galileo's hypothesis that gravity will cause bodies of differing masses to fall at the same rate in a vacuum, by dropping a hammer and a feather.