The Schwarzchild manifold of general relativity theory is unsatisfactory as a particle model because the singularity at the origin makes it geodesically incomplete. A coupling of the geometry of space-time to a scalar field φ produces in its stead a static, spherically symmetric, geodesically complete, horizonless space-time manifold with a topological hole, termed a drainhole, in its center. The coupling is Rμν=2φ,μφ,ν; its polarity is reversed from the usual to allow both the negative curvatures found in the drainhole and the completeness of the geodesics. The scalar field satisfies the scalar wave equation □φ=0 and has finite total energy whose magnitude, expressed as a length, is comparable to the drainhole radius. On one side of the drainhole the manifold is asymptotic to a Schwarzschild manifold with positive mass parameter m, on the other to a Schwarzschild manifold with negative mass parameter m¯, and - m¯ > m. The two-sided particle thus modeled attracts matter on the one side and, with greater strength, repels it on the other. If m is one proton mass, then - m¯/m ≈ 1+10-19 or 1+10-39, according as the drainhole radius is close to 10-33cm or close to 10-13 cm; the ratios of total scalar field energy to m in these instances are 1019 and 1039. A radially directed vector field which presents itself is interpreted, for purposes of conceptualization, as the velocity of a flowing ``substantial ether'' whose nonrigid motions manifest themselves as gravitational phenomena. When the ether is at rest, the two-sided particle has no mass on either side, but the drainhole remains open and is able to trap test particles for any finite length of time, then release them without ever accelerating them; some it can trap for all time without accelerating them. This massless, chargeless, spinless particle can, if disturbed, dematerialize into a scalar-field wave propagating at the wave speed characteristic of the space-time manifold.