Several centuries after the events of Second Foundation, two citizens of the Foundation seek to find Earth, the legendary planet where humans are said to have originated. Interestingly, even less is known about Earth than was the case in Foundation, when scholars still seem to know the location of 'Sol'.
The story follows on from Foundation's Edge, but can be read as a completework in itself. (It does, however, give away most of the mysteries that Foundation's Edge is built around.)
Part I: Gaia
Councilman Golan Trevize, historian Janov Pelorat, and Blissenobiarella of the planet Gaia (all of whom were introduced in Foundation's Edge) set out on a journey to find humanity's ancestral planet — Earth. The purpose of the journey is to settle Trevize's doubt with hisdecision at the end of Foundation's Edge to embrace the all-encompassing supermind of Galaxia.
Part II: Comporellon
First, they journey to Comporellon, which claims to be the oldest currently-inhabited planet in the galaxy. Although many other planets make that claim, Comporellon has a very long history with which to back it up. Upon arrival, they are imprisoned, but negotiate their way out. Whilethere, they find the coordinates of three other Spacer planets. Since the Spacers were the first colonists from Earth back in the ancient days of space travel, it is surmised that their planets would be fairly close to Earth.
Part III: Aurora
The first Spacer planet they visit is Aurora, where Trevize is nearly killed by a pack of wild dogs, presumed to be the descendants of household pets longsince reverted to wolf-like savagery. They escape when Bliss manipulates the dogs' emotions to psychologically compel a retreat.
Part IV: Solaria
Next, they go to Solaria, where they find what the Solarians — who have survived the Spacer-Settler conflicts by clever retreat detailed in Asimov's novel Robots and Empire — have evolved into self-reproducing hermaphrodites, who have remained generallyintolerant of human physical presence or contact. They have also modified themselves to have a natural ability to mentally channel ("transduce") great amounts of energy, and utilize this as their sole source of power. The Solarians intentionally avoid ever having to interact with each other, except by holographic apparatus ("viewing"), and reproduce only when necessary to replace someone who hasdied. Bliss, Pelorat, and Trevize are nearly killed by a Solarian named Sarton Bander. Bliss, however, deflects the transducer brain-lobes at the moment Bander tries to use them to kill. Bliss intends to knock out Bander, but has not had sufficient time to learn the full workings of the transducer and accidentally kills him instead. While escaping, they find what they assume to be Bander's immaturechild, Fallom, in a state of panic because its robotic nursemaid, like all other robots on the estate, has stopped functioning. The child Fallom cannot inherit the Bander estate, as would normally be the Solarian custom, because it is too immature to be able to use its transducer lobes. There being no other place for the child on Solaria, the decision of the robots who immediately arrive toinvestigate the loss of power is that Fallom is to be killed. Upon learning this, Bliss insists that they take Fallom with them.
Part V: Melpomenia
They next go to Melpomenia, the third and final Spacer coordinate they have. They find that the atmosphere has become depressurized to a few thousandths of normal atmospheric pressure. Wearing space suits, they enter a library, and find a statue with thecoordinates of all of the Spacer worlds. While departing Melpomenia, they notice a carbon-dioxide-feeding moss has begun feeding off insignificant leakages in their space suits. Barely recognizing this before stepping on their fully pressurized ship — which would have likely been disastrous — they set their blasters to minimum power to fry it off, and then set the ship to heavy UV-illumination...