The episode details Dr. Otto Octavius as an accidental laboratory incident caused by the Green Goblin where the radiation merges special mechanical tentacles to his skin and seemingly damaged his brain (although it was later suggested that what was interpreted as brain damage was in fact his mind rewiring itself to accommodate four extra limbs), and turns him from a timid and weak scientist into the villain Dr. Octopus.

Unbeknown to him, the villain Green Goblin sneaks into the lab, activating the chamber's generators while locking Octavius inside.

They reach Coney Island through their ensuing brawl and Octopus, aware of Spider-Man's desire to save innocent lives, kidnaps Liz.

With only an hour of energy left, he climbs to the top of a roller coaster, forcing Spider-Man to choose between the power pack and Liz.

[4] The episode's title, "Reaction," expands the series theme "The Education of Peter Parker" chosen by developer Greg Weisman.

To meet these expectations, he followed a simplistic strategy—"be mindful of the history, listen to the direction and guidance of Greg Weisman, and then write an episode with a good, entertaining story that is packed with as much action and humor as 22 minutes will allow."

MacNicol chose to base Dr. Octopus' voice on that of Laird Cregar, a 1940s actor who, throughout his short-lived career, was challenged with obesity, until he lost too much weight and died at age 30.

Eric Goldman of IGN, giving the episode an 8.2. rating ("Impressive"), wrote favorably, noting that the depiction Dr. Octopus "was very clever – on one hand, he was a pitiful, cowering pushover.

Goldman applauded both Dr. Octopus' line, "I've been good" and when Spider-Man crashes into several stuffed animals, only to be pulled out by Dr. Octopus in a manner similar to a Claw crane; he concluded that, "This show continues to leave several prime time live-action shows to shame when it comes to building serialized stories and has been one of the most pleasant surprises of the TV season.

"[6] Luke Bonanno of Ultimate Disney listed "Reaction" as one of his top five favorite episodes of the first season, while commenting that "the uniform excellence of the lot [makes] this a challenging task.

"[9] Rob M. Worley of Mania described Dr. Octopus' introduction as "explosive," while praising MacNicol for balancing such a heavy workload between both the episode and other series.