Pauline (Pearl White), her fiancé Harry, and her evil greedy guardian Koerner meet a Lt. Summers. He is a lieutenant in the navy and also designs submarines. Lt. Summers invites Harry, Pauline, and Koerner back to his place to meet his fiancée Mlle. Yagow.
It turns out that Mlle. Yagow is a spy along with the valet. Koerner recognizes Mlle. Yagow as a possible ally in his evil plot to get rid of Pauline. He offers her $100,000 upon Pauline’s death.
Lt. Summers completes his submarine plans and places them in what he believes is a secret place. The valet comes goes into the office not long after Summers has walked out. He finds the plans and takes them to his superiors. The valet tells his superiors he will do whatever it takes to sink the submarine with Summers in it even if it means he will die as well.
Koerner arranges for Pauline to go on the submarine knowing full well what the valet has planned. The submarine goes underwater and as soon as it reaches a low depth the bomb goes off sinking the vessel to the ocean floor. The valet is found hiding and confesses about him and Mlle. Yagow working for the enemy who wants their plans. The submarine crew are able to get Pauline out and to safety by having her crawl through the torpedo launcher.
Pauline and Harry set up a rescue for the crew. They then go find Mlle. Yagow. They fight her and the leader she is working for and escape with the plans.
The submarine is able to be lifted to the surface and the crew are saved.
Finally, Pauline is some peril! I liked The Tragic Plunge a lot. It was a little nerve wracking.