Recently a demonic horde lead by N'astirh has invaded New York. Due to the dimensional tear, an usual type of energy is let loose upon New York, affecting people in a variety of ways. The X-Teams are handling this problem, so don't worry about that.
Years ago (in Marvel time) Harry Osborn, under the influence of LSD discovered that his father had killed Gwen Stacy, Peter's then-girlfriend, and subsequently died during a battle with Spider-Man in a back alley. He changed his father's clothes to conceal his dual identity from the police, discovered Peter was Spider-Man, and eventually became the second Green Goblin to avenge his father's death. He attempted to kill some of their closest friends before he was stopped and sent to a mental institution. Eventually he developed partial amnesia, blocking out his brief career as a costumed criminal and Peter's secret identity. He later married Liz Allen and had a son named Normie (told you it was a long time).
Recently (again) the second Hobgoblin has attacked Harry at his house demanding he turn "it" over to him. Harry bought some time by telling him "it" was in the downtown office of Osborn Chemical, having no idea what he was talking about. Hobgoblin promised to return if he was lying.
The combination of the dimensional energy and the stress brought on by the Hobgoblin's attacks have begun to restore Harry's memories of his time as the second Green Goblin. As of this point, Harry remembers the location of some spare Goblin equipment he had hidden in his attic.
Harry's memories of his stint as the second Green Goblin come flooding back as he suits up. He knows he and his family are living on borrowed time until the Hobgoblin discovers his ruse. Unwilling to rely on anyone's help, Harry hops on the glider and leaves for Osborn Chemical to confront the Hobgoblin.
As Peter makes his rounds to the Daily Bugle, E.S.U., and back to Bedford Towers, he receives a message from Liz Osborn informing him that Harry's missing and she - correctly - believes he went to confront the Hobgoblin. Remembering his bluff from earlier that morning, Peter heads to Osborn Chemical.
When he arrives he finds the Goblins going at it full force: pumpkin bombs, sparkle blasts, razor bats, and everything they could fit into their respective weapons cache. Realizing it's two against one, Hobgoblin leaves, only to be followed by Harry and then by Spider-Man hitching a ride on Hobby's glider. Noticing the extra weight, Hobgoblin quickly changes direction, slams Spider- Man into a nearby water tower, and gets out of range quickly. He then focuses his attention on Harry.
Harry is eventually able to outmaneuver the Hobgoblin and drops all remaining pumpkin bomb on top of him, causing him to lose control and crash the glider into a nearby rooftop. When he finally has the advantage, Harry forces Hobgoblin to admit why he was threatening him. He responds "the Goblin formula". The strength-enhancing compound that gave the first Goblin super strength. Harry informs him that the formula was lost years ago otherwise he would have used it against him. Furious that this was all for nothing, Hobgoblin fires randomly at him to cover his tracks and leaves.