Ideal for students who may have little previous experience with abstraction and proof, this text provides a rigorous and unified—yet straightforward and accessible—exposition of the foundations of Euclidean, hyperbolic, and spherical geometry. Unique in approach, it combines an extended theme—the study of a generalized absolute plane from axioms through classification into the three fundamental classical planes—with a leisurely development that allows ample time for students' mathematical growth. It is purposefully structured to facilitate the development of analytic and reasoning skills and to promote an awareness of the depth, power, and subtlety of the axiomatic method in general, and of Euclidean and non-Euclidean plane geometry in particular.