What follows is a fast paced character rich story, which feels like a slicker, updated version of an old, 50s SF movie. Timothy Zahn takes the well worn idea of aliens among us, and gives it a few new tweaks. Also injected into the mix is something not often seen in SF literature, the realistic portrayal of a marriage. The Whittier's have not been getting along for some months, but now they have more pressing matters to deal with, and a mystery to unravel. Among those who also become involved are an aged but spry former Ellis Island clerk, Otto Velovsky, and New York Detective Thomas Fierenzo. It is Fierenzo who believes that a gang war is about to break out, between the Greens and the Grays, and he is determined to stop it. As the Whittier's and Fierenzo work to protect Melantha, and their city, they are gradually immersed in the cultures of the two groups. Greens and Grays are both able to pass for human, and unknown to each other, both arrived in the United States as refugees from a war they each claim the other started. For almost three quarters of a century, neither Greens nor Grays realised that their old enemies had also come to Earth. Until a young Grey boy became friends with a young Green girl. Unfortunately, she has a rare and dangerous talent, which has almost tipped the two sides into urban warfare.

The Grays have an affinity with electronics and construction, plus the ability to climb up walls unaided. Some among them have advanced communications devices, and are armed with sound-based weapon-tools, called hammer guns which work like Thor's hammer. The Greens have a kind of telepathy, and are able to emit a debilitating shriek. They can also hide or sleep inside trees, like the wood nymphs of legend. Green society is formally aligned according to special talents, which manifest at puberty. Some Greens possess special devices called trassks, which appear to be large brooches, but can be mentally commanded to fashion themselves into a variety of objects such as knives, throwing disks and simulated guns. Both sides have integrated into human society, but only on the surface. Privately, the Greens and the Grays keep strictly within their own communities, obedient to their ancient hierarchies, and ruled by cultures based on paranoia and racial hatred. The human cast have to contend with their alien friends ingrained lack of trust, as well as working out who is telling the truth, and trying to keep Melantha Green from harm.

There's always a lot going on, with around 30 major characters, but Zahn is careful to keep his main players at the centre. The mystery of who the Greens and Grays really are, and where they came from, was just as interesting as their present. A few holes in the back story made me frown. For example, both groups arrived on Earth in the mid-1920s, using what are sparsely described as transports. These large vessels have supposedly remained hidden underwater, along the shoreline. The excuse given for not moving or using them for anything more than hydroponics and storage, is that they'll show up on human sonar. I found it hard to believe that any ship capable of moving through space or time would not be equipped with an ECM suite. Similarly, the history of how the war between Greens and Greys began, is about as flimsy as a paper house. I also had difficulty believing that two alien groups, physiologically different to humans, could live in the heart of a city like New York, for decades and post-9/11, without a single one being detected. But, leaving these issues aside, when The Green And The Gray is in the present, it hums along like a well oiled machine, purring whenever the topic is relationships, and changing into high gear for the action scenes. All divided into inviting chunks of fluent, breezily written text, which are ideally sized for reading on the way to work, or last thing at night.