Twenty-sixth-century alien-contact yarn, second in the trilogy begun with last year's paperback Threshold. Space scavenger Keebler finds a huge, mysterious, glowing Ball in space and tows it back to Threshold, humanity's base in the asteroid belt and portal to interstellar travel. Among those who suspect that the Ball's purpose is to facilitate alien contact is Joe South, a 21st-century test pilot who returned from an experimental voyage only to find that 500 years had elapsed; during his travels, he glimpsed the aliens but cannot now remember. After administrators, industrialists, and security chiefs wrangle over the Ball, the aliens--representing a galactic Unity of intelligent species--finally show up; administrator Mickey Croft meets their leader, the Interstitial Interpreter, and is profoundly changed by the experience--the aliens, you see, are dissociated from linear time. But since the humans cannot agree on a policy, the aliens withdraw, leaving the Ball as a portal for further contact; South recovers his memories, and Croft's new nonlinear awareness spreads to others at Threshold. Intriguing time/mind notions and quite promising aliens are undermined by wretchedly feeble plotting and threadbare characters: overall, no better than mediocre.