Prince Karl Heinrich (Novarro) is the crown prince of a strongly Germanic kingdom called Karlsburg note . He has grown up in the court of his uncle the king, living a lonely, cheerless life. His only friend is his avuncular tutor, Dr. Juttner (Jean Hersholt). After reaching adulthood Karl Heinrich goes off to study at the military college, Heidelburg, with Dr. Juttner as an indulgent chaperone. There he gets to have fun for the first time in his life, discovering the joys of beer drinking and boisterous fraternity life, as well as finding love with Kathi, the pretty, good-hearted barmaid (Shearer). Yet the pressures of court life loom back home, threatening to destroy Karl Heinrich's happiness and chance at love.
The Student Prince in Old Heidelberg was based on the novel Karl Heinrich (1898) and the stage play adaptation, Alt-Heidelberg (Old Heidelberg, 1901), both by Wilhelm Meyer-Förster. It also owes a debt to the highly successful operetta version of the story by Sigmund Romberg, The Student Prince, which had debuted on Broadway to great acclaim in 1924. (The film does not use any of the operetta's music.) The Student Prince in Old Heidelberg bombed when it ran in theaters but is recognized today as a career highlight for Novarro and the first great film directed by Lubitsch.
- Arranged Marriage: The pressure on Karl Heinrich to enter into an Arranged Marriage with a princess from another royal house is what brings an end to his romance with Kathi. Although a king probably couldn't marry a barmaid in any case.
- The Chains of Commanding: Karl Heinrich's duties as crown prince (and later king) of Karlsburg prevent him from having a happy life.
- Description Cut: Kids look at Karl Heinrich's picture in the window and say "It must be great to be a prince." Cut to Karl Heinrich, inside the palace, unable to play with other children.
- Downer Ending: Karl Heinrich loses forever the comradeship of his fellow students and will never see Kathi again. And just to make it more depressing, Dr. Juttner dies.
- Fun with Subtitles: Fun with intertitles, that is. One scene has an emotional greeting between Kathi and Karl Heinrich. She says "KARL HEINRICH!" and the title card blows up until the words fill the screen from side to side. Then he says "KATHI!" and the same thing happens.
- Gilded Cage: The palace is this for Karl Heinrich. One shot shows him literally behind bars (the fence around the palace), watching other kids outside having fun kicking ball.
- Gray Rain of Depression: It is pouring rain when Karl Heinrich returns to Karlsburg, having been forced to leave Heidelburg due to his uncle's fatal illness.
- Have a Gay Old Time: "...gay, romantic Heidelberg."
- Lonely Rich Kid: Justified, in this instance, as Karl Heinrich is the crown prince of Karlsburg, growing up in his uncle's castle with no other children to play with.
- Long Title: The original play was called Old Heidelburg, and the hit opera was called The Student Prince, so the titles got put together. Interestingly, the title card to the surviving film print simply calls it Old Heidelberg, but most every reference refers to it by this longer title.
- A Minor Kidroduction: The film begins with Karl Heinrich as a small boy, coming to live at his uncle's court, completely terrified. He then lives a life completely devoid of childhood fun until Dr. Juttner arrives and becomes his friend and surrogate father. Then after a Time Skip, Ramon Novarro enters the picture.
- Private Tutor: Karl Heinrich, the crown prince of a little Ruritania-style German principality, receives a Private Teacher type of tutor, Dr. Juttner, who proceeds to become the little boy's only friend.
- Reality Ensues: The illness and death of the king brings a sudden, screeching halt to Karl Heinrich's carefree student life and his romance with Kathi.
- Running Gag: All the saluting, bowing, and doffing of hats.
- Ruritania: A vaguely Germanic setting in 1901.
- Smoking Is Cool: Played completely straight, and with a big dose of Values Dissonance for a latter-day viewer. One of the things Dr. Juttner does to bring some fun into Karl Heinrich's life is introducing him to smoking, complete with a scene where Karl Heinrich has fun blowing smoke out of his nose.
- Stranger in a Familiar Land: After becoming King, Karl Heinrich comes back to Heidelberg to find out that nothing is the same anymore. The beer hall where he had such good times is no longer a student hangout. His old fraternity mates behave in a stiffly correct manner towards him now that he's king (the pang of regret on his friend's face when Karl Heinrich turns his back is a heartbreaking moment). Even the flower-bedecked hillside where Karl Heinrich and Kathi kissed is now barren in winter.
- Time Skip: At least ten years, from Dr. Juttner's arrival to college-age Karl Heinrich taking his oral exams.