From what research I have done, this episode was not shown on its intended air date in the third season. It may have been shown during syndication, but I can't confirm that.
At any rate, this is a little different than normal. We have another episode that was lifted from the "Rocket Robin Hood" series, substituting Spider-Man for Robin.
Mr. Mxyzptlk does not appear in this episode either.
The evil dictator Infinata of Dementia-Five learns that he is being watched by two scientists from the planet Gorth in the Kabazar galaxy. He doesn't like this in the least because they have uncovered his secrets and attacks them in retaliation.
The harmless beings know that their days are now numbered. They have collected data for centuries and do not want their efforts to be in vain. One scientist decides to run the extensive Library of Gorth containing their work through a computer that converts the information into energy which is then stored in a special containment module. [Advanced beings writing everything down by hand. Good thing they're immortal.] Knowing that time is short Athom says goodbye to his assistant, boards a rocket, and escapes just as the planet begins to crumble. [Dude! That rocket was big enough for both of you. Also this smells of Superman's origin.]
Using his "ultra sensory perception" Infinata discovers Athom has escaped with the Libarary of Gorth. He dispatches his army after him to destroy them both. Athom makes his way to Earth and is transformed into a meteor along the way. His arrival attracts the attention of Spider-Man.
Being a curious sort, he follows the shooting star to its crash site. He is quite surprised to find that the pilot - Athom - is only a few inches in height [Much like a Smurf, only uglier.]. The tiny alien communicates telepathically with Spider-Man, explaining the situation as quickly as he can before he dies. He explains that Dementia-Five is everywhere and nowhere at the same time. [Whoa, man, trippy]
Spider-Man decides to take the tiny sphere to the United Intelligence Agency. Hopefully they will be able to decode it [No! Repeat after me: Baxter Building]. He places the sphere in a special ring that mysteriously appeared and makes his way to the UIA. [Ok, a dying alien crashes on Earth and gives an unsuspecting schlub a ring that contains a great source of power. Green Lantern, anyone? Also what did he do with Athom?]
Infinata uses his vast powers to observe this transaction and will have none of this. He brings Spider-man to Dementia-Five and demands he hand over the encoded Libarary of Gorth. When Spider-Man refuses, Infinata uses his X-ray powers to scan Spider-Man. Unable to find the sphere, Infinata decides to kill him by suffocating him in the substance of the dimension.
When Spider-Man briefly closes his eyes, he finds that he's back in New York. Realizing that Dementia-Five is a dimension of illusions, he wills himself back to reality and breaks Inifinata's hold on him. Just in time too; he was falling toward his death in reality. Spider-Man stops his fall and takes a moment to compose himself.
Infinata decides to conquer other galaxies as they now have his secrets and can stand up to him.
He pulls the data sphere from inside his web-shooter, thankful that Infinata didn't examine him too closely. He then leaves to deposit this with the proper authorities.
They ripped off another episode of "Rocket Robin Hood" which contained the partial origin of Green Lantern to make a Spider-Man cartoon that is much more appropriate for a Dr. Strange series. That is a confusing - yet accurate - description of the episode.
All they're missing is the bat-signal.
1/2 web, but only in part for the reasons you are thinking.
The concept of a dimension-hopping Spider-Man without a guide doesn't work. This has been mentioned before. The biggest problem I have is that an entire episode from a different series was used as the basis for this episode. I might go a bit easier on them if something more appropriate than "Rocket Robin Hood" was chosen. This stands out as nothing more than a poor decision by the animation company to churn out another "new" episode with little effort.
On a positive note, had they done this for the Fantastic Four 1967 series - which I know was created by Hanna-Barbera - it would have made more sense. I say this to emphasize that it's not only the story source that bothers me, but also the pairing of story and character.
The boundaries of Dementia-Five were ill-defined, making Spider-Man's escape from it both easier to achieve and less satisfying in terms of story resolution.
With all the powers at Infinata's disposal he couldn't find the data sphere in his web-shooter? Way to miss the boat on that one, guys.