“For he that diligently seeketh shall find; and the mysteries of God shall be unfolded unto them, by the power of the Holy Ghost, as well in these times as in times of old, and as well in times of old as in times to come; wherefore, the course of the Lord is one eternal round.” -1 Nephi 10:19
The Perpetual Plan is “circular” in its direction and structure. Circular because it is designed around un-ending propagation. Eternal parents produce spirit offspring, the Perpetual Plan is adopted through a family council, spirit children obtain physical bodies to gain earthly experience to progress toward perfection themselves and ultimately realize their divine destiny as heirs of Eternal Lives–to become like their Parents.
(The following diagram was created while I was teaching a temple preparation class to BYU freshmen, and is based on the content of Lesson 1 from the LDS temple preparation course manual. The intent was to teach them the concept of the actual multi-dimensional Plan. We often teach a “two-dimensional, stick-figure” style Plan of Salvation, which isn’t very exciting, and grossly inaccurate, IMO.)