The book and film series focused on the exploits of Francis, an experienced Army mule, and Peter Stirling (played by Donald O'Connor), the young soldier whom he befriends (and stays with through civilian life and then back into the military).
As the titles indicated, each film had a different setting or gimmick, exposing the world-wise mule and the naive GI to race track excitement, the world of journalism, and many branches of the military, from West Point to the WACs to the Navy.