"Marvel Super-Heroes #13" was published in 1968 and features the first appearance of Captain Marvel, a character who would become significant in the Marvel Universe. The issue showcases the origin story of Mar-Vell, a Kree warrior who comes to Earth and ultimately becomes a hero. In this comic, Mar-Vell is tasked with observing Earth but finds himself conflicted about his mission. The story introduces readers to the Kree-Skrull conflict and sets the stage for future adventures involving Captain Marvel and other key characters in the Marvel Comics universe.