train nltk classifier for just one label
You can simply train a binary classifier to distinguish between sci-fi and not sci-fi
So train on the movie plots that are labeled as sci-fi and also on a selection of all other genres. It might be a good idea to have a representative sample of the same size for the other genres such that not all are of the romantic comedy genre, for instance.
I am just starting out with nltk, and I am following the book. Chapter six is about text classification, and i am a bit confused about something. In the examples (the names, and movie reviews) the classifier is trained to select between two well-defined labels (male-female, and pos-neg). But how to train if you have only one label. 
Say I have a bunch of movie plot outlines, and I am only interested in fishing out movies from the sci-fi genre. Can I train a classifier to only recognize sci-fi plots, en say f.i. if classification confidence is > 80%, then put it in the sci-fi group, otherwise, just ignore it.
Hope somebody can clarify, thank you,