As far as stories go, this is a forgettable tale where the team fights a body-possessing villain known as Space Phantom, who enters the bodies of each member causing lots of conflict and misunderstanding, until he tries to enter Thor. The Asgardian’s physiology rejects Space Phantom, and he is vanquished.
It’s used as the excuse for Hulk to leave the team. They didn’t really need an excuse for that, did they? Kind of a deus ex machina. Which is how Space Phantom would be used in the future, as a source of conflict and an explanation for weird, uncharacteristic behavior.
Frankly, if I were Hulk, I would have left because being in The Avengers messed with his body. Here’s his foot from the splash page:
Creators: Lee and Kirby
First appearances: Space Phantom