The story arc is mostly remembered for the striking aesthetic of Spider-Man, swinging through the city with four extra arms.
Spider-Man has had enough of being Spider-Man. Peter feels miserable about all the anguish he had to endure. His best friend Harry Osborn has become a junkie, and his close friend Captain George Stacy died in his arms. His daughter Gwen Stacy - Spidey's girlfriend - wrongly blamed Spider-Man for this. Peter has a fatal thought: for Peter Parker to live, Spider-Man must die!
So, Spidey brews up a chemical cocktail which is intended to exterminate his spider powers. Peter takes the cocktail and falls into a troubled sleep, in which he fights all his enemies while enduring excruciating side pains. When he wakes up, he notices a ghastly thing: he has six arms! The potion increased his spider powers rather than to nullify them!
He then visits his only possible confidant, Dr. Curt Connors, alias the Lizard, and fight a vampiric villain called Morbius. Using Morbius' blood, both brew an antidote that turns Spidey back into normal.