Steven, along with nearly five hundred other humans, has been offered the opportunity of a lifetime. An advanced alien species known as the Pic have proposed a test to see whether humans and Pic can bring their two societies together for mutual benefit. Steven will spend the next thirty years of his life on the Pic home world, first studying at a university created for joint human-Pic classes, then living and working in the Pic community. For two years, Steven's been engaged in worldwide competitions for a candidate slot, and against enormous odds, he succeeds.

But Steven and his new Pic friends have discovered something worrying... disasters have recently occurred both on Earth and Whistlestop, and no one seems to be able to identify the reasons behind them. One of Steven's classmates has concerns about the portal technology the Pic use for transport, that their species found thousands of years earlier but did not create - does this make them vulnerable in ways that they cannot predict? Are the recent attacks connected with the original inventors of the portals? This becomes the true test of the fusion trial: Can a small, joint team of humans and Pic defeat a hidden enemy that has the power to destroy worlds?