While attempting to investigate a strange disturbance in space, Reed Richards, his wife Sue, her brother Johnny Storm, and Reed's best friend Ben Grimm were bombarded with cosmic rays.
The accident altered their bodies in strange ways, Reed could now stretch and mold his body into any shape he pleased; Sue had the ability to turn herself and objects around her invisible; Johnny could create and manipulate flames and Ben was permanently turned into an large, orange rocked creature with superhuman strength. Together, with their newfound powers, they began battling evil and exploring the unknown.
Thing and Hulk had a long running grudge by the time Hulk and She-Hulk returned to New York. Bruce Banner hoped to find Reed Richards and ask for his help with a cure before The Leader's intervention caused him to Hulk out.
Ben, lonely after being separated from Alicia Masters, was uninterested in She-Hulk, despite her best efforts. She and Ben managed to defeat Leader's minions, and Hulk and Thing settled their differences.
In the Secret Wars, Spider-Man realized that there was an opponent he knew nothing about, Doctor Doom. So he called upon the Fantastic Four, who had previously defeated Doom several times. After some disagreements, they later agreed to help Spider-Man.
They later separated, with the Four launching an attack on Doom by themselves. Upon traveling, Ben was kidnapped by one of Doom's Doombots. They later learned that Doom had no intention of harming Ben, instead he cured him by creating a device which gave him the ability to turn back into Ben Grimm. He did this so that Ben would tell him why they were transported to the alien planet. After being told, Doom transported to Beyonder and used the same device he cured Ben with to steal Beyonder's powers.
Ben later used the same device to separate Doom from Beyonder's powers and Beyonder sent everyone back to earth. However, they were all left with no memory or evidence that the war ever occurred, thereby leaving Thing without the device of changing back to Ben Grimm.
In the Comics
Reed and Sue married after obtaining their powers, not before.
Others seen in the Marvel Animated Universe who have joined the team at one time or another in the comics include Ghost Rider, Spider-Man, Medusa, Storm, Black Panther, Namor, She-Hulk, Hulk, and Wolverine.