Begins: http://ficlets.ficly.com/stories/168

Nina notices a strange terrarium on her co-worker Alan’s desk. It seems bigger on the inside than the outside, and has strange creatures in it that he claims are unicorns. When Nina shows a bit too much curiosity, Alan traps her inside of it.

The terrarium turns out to contain a tropical island, and the unicorns look much more like real unicorns than they did through the glass. Prompted by the “gift” of a book on shelter-building, Nina builds a shelter to wait out the rainy season.

After Nina rescues a trapped unicorn, the rest of the herd transform her into a unicorn like them—but more, make her their Queen. They also reveal that Alan had trapped three other girls from the secretarial pool before her, who had also been changed. Nina chooses to adopt the unicorn form full-time.

Outside the terrarium, Alan can no longer find Nina, and becomes upset that yet another prisoner has mysteriously vanished. He plans to enter the terrarium himself, to investigate.