Foundation's Edge

After the defeat of the Mule by the Second Foundation, Terminus enjoys a period of prosperity and stability which is publicly attributed to the Seldon Plan, but which some feel is the work of the Second Foundation. A search is begun to determine if the Second Foundation still exists. Meanwhile, the Second Foundation finds that there is evidence of an independent force acting against the Mule to protect the Seldon Plan. A search is launched to determine what this force is and if it is a threat. The end result is a search for Earth (Terminus being the last planet settled by the galactic empire and Earth being the metaphoric opposite of first human planet settled). A final three way confrontation results in a fateful decision and an open question.

Synopsis from Open Library.