The single-player campaign in nail'd is comprised of multiple race series in which you must finish first or second in order to advance through each series in order to move on to the next. Each series increases in difficulty to the point where after the first few series, I had to race some courses multiple times in order to consistently finish in the top 2. The increases in difficulty are gradual enough that my skill seemed to progress along with the courses. http://naildbiter.com/FqdDH