A Union Cavalry outfit is sent behind confederate lines in strength to destroy a rail supply center. Along with them is sent a doctor who causes instant antipathy between him and the commander. The secret plan for the mission is overheard by a southern belle who must be taken along to assure her silence.

Dexter Riley is a science student at Medfield College who inadvertently invents a liquid capable of rendering objects and people invisible. Before Dexter and his friends, Debbie and Richard Schuyler, can even enjoy their spectacular discovery, corrupt businessman A.J. Arno plots to get his greedy hands on it. Slapstick hijinks ensue as Dexter and his pals try to thwart the evil Arno before he can use the invisibility spray to rob a bank.