Grimm's freshman year roommate was the brilliant science student Reed Richards, who became his closest friend. During their first meeting Richards confided in Grimm his intention someday to build a starship for interstellar travel. Grimm jokingly promised that he would pilot the starship for Richards if he ever built it.

Richards' project was based in Central City, California. When the federal government threatened to withdraw its funding from Richards' project, Richards decided to take the starship on a test flight himself as soon as possible. Grimm was opposed to the idea, warning that the starship's shielding might prove to be inadequate protection from intense radiation storms. Nevertheless, Grimm was persuaded to serve as pilot, and Richards' future wife Susan Storm and her adolescent brother Johnny insisted on accompanying Richards' as passengers. The four friends stole onto the launch facility, entered the starship, and launched it. They intended to travel through hyperspace to another solar system and back. However, unknown to Richards, a solar flare caused Earth's Van Allen radiation belts to be filled temporarily with unprecedented (as far as is known), ultra-high levels of cosmic radiation. Since the ship was designed to shield against ordinary levels of radiation, the cabin interior was subject to intense cosmic ray bombardment which irradiated the four passengers and wrought havoc on the ship's controls. Pilot Grimm was forced to abort the flight and return to Earth.

Once back on Earth, the four passengers discovered that the cosmic radiation had triggered mutagenic changes in this bodies. Grimm was transformed into an orange colored, thick-skinned, heavily muscled, and superhumanly strong "thing." Richards convinced the three others that the four of them should use their new powers for the good of humanity as members of a team he named the Fantastic Four. Richards called himself Mister Fantastic, Susan and Johnny Storm took the names of the Invisible Girl and the Human Torch, and Grimm, morose over his new grotesque appearance, named himself the Thing. Under Richards' leadership the Fantastic Four has become Earth's most honored team of superhuman adventurers, and has saved the world from conquest or destruction many times.