Worlds of the Wall

Zeke Bolivar slipped out of null space to find a planet which defied, just by existing, all natural laws. For it wasn't a sphere, but only half of one. It floated in space, circling in a calm, regular orbit around its sun, like a vast half-grapefruit, cut neatly down the middle. Rimming the grotesque half-palnet along the "cut" was an opaque black wall. When he landed near the edge of the hemispheroid planet, he found he was expected. As events became more and more bizarre, he considered that he might simply be mad. Then an oddly familiar old man shoved Zeke at the black wall - which turned insubstantial and let him fall through...