Silas Mortimer Mulch is an ecological supergenius, with a laboratory greenhouse holding the keys to countless scientific breakthroughs. But when his niece and nephew, Selena and Tommy, come to visit Silas and find him missing, his organic computer SOFIA tells them that he is lost within the ever-expanding and constantly changing greenhouse. Donning hi-tech greenhouse suits, Selena and Tommy must go in after Silas, and are soon confronted with astonishing nanotechnology, fungi that act as hard drives of information about the greenhouse, and plants that can create duplicates of people they encounter.