The Flash [Barry Allen] (introduction, scientist, origin); The Flash [Jay Garrick] (image only); Iris West (introduction); The Turtle Man [aka later as The Turtle] (villain, introduction)
By a freak accident involving lab chemicals and lightning, Barry Allen becomes the fastest man alive and originates a costume based on the comic adventures of the Flash from the 1940's. And it's a good thing too: for he must now face the Turtle Man, termed "The Slowest Man Alive".
The criminal Mazdan was tried for his crimes and sentenced to be sent to a future time period where he could harm nobody but was accidentally sent backwards to the 20th century, where he is captured by the Flash.
Flash faces off against Captain Cold, who commits crimes by using a cold-gun to freeze things and create mirages. But once Flash figures out what Captain Cold can do, he ignores the mirages and takes the villain in.
Barry Allen's friend Fred Pearson, star of the Central City Theatre's "The Great Gorilla," has been having blackouts at the same time that a mysterious gorilla has been seen roaming the city. Flash must solve this mystery and that of a super-sonic vehicle that passes him like he's standing still. Its pilot, Gorilla Grodd, has come to find Solovar, who is being held at the Central City Zoo. Grodd steals Solovar's "force of mind," and begins his plan to take over Gorilla City. But Solovar finds Barry Allen, explains the situation, and the two of them head to Gorilla City to confront Grodd.