Once it finishes, there will be a video file in your Downloads folder (~/user/Library/Downloads), named whatever the torrenter named it. It will most likely have a .mov or .wmv ending, as that is the file type. Double click on it, and it will open an application called Quicktime, which it will play in. QUality might not be the best, but it will work.
While the torrent is going, see if Hulu (www.hulu.com) or Netflix has it.