The kids are on a South American expedition with Velma's uncle, Professor Cosmo Dinkley, to recover the Celestial Orb, a baseball-sized ruby, that has a map to the Lost Temple of Sirius. The temple is believed to have been built by the ancient astronauts and filled with secrets and treasure. It soon seems that they are not the only ones searching; Professor Dinkley's ex-assistant Harriet Mullins is lurking in the background, waiting to steal the orb.