In an alternate 1960s reality known as Earth-828, Dr. Reed Richards and his wife, Dr. Sue Storm, discover that they are expecting their first child. The news comes as the Fantastic Four are preparing to celebrate their fourth anniversary as the world’s most famous superheroes. Reed and Sue, along with Sue’s younger brother Johnny Storm and Reed’s longtime best friend Ben Grimm, became superhuman four years earlier after being exposed to cosmic radiation during a space expedition aboard the Excelsior. Reed gained the ability to stretch his body, Sue developed the power to manipulate invisible energy and create force fields, Johnny became capable of controlling fire and transforming into the Human Torch, and Ben was transformed into a massive, rock-skinned being with extraordinary strength. Living together in the Baxter Building, the four have embraced their roles as both superheroes and family, defending Earth from threats including Giganto, the Mole Man, the Mad Thinker, and the Red Ghost and his Super-Apes.

As Sue’s pregnancy becomes public knowledge, she temporarily steps away from her work with the Future Foundation, an organization dedicated to global peace and demilitarization. The announcement generates widespread excitement and speculation about whether the unborn child will inherit the Fantastic Four’s powers. Meanwhile, Reed becomes increasingly fascinated by the possibility of returning to space. With assistance from his robotic laboratory companion H.E.R.B.I.E., he studies old spacesuit designs and continues developing his experimental Teleportation Bridge, a device capable of transporting matter across enormous distances. During one experiment, Reed successfully teleports organic material, although the process consumes tremendous amounts of energy. At the same time, he studies transmissions from deep space and monitors the development of his unborn child, unaware that a far greater cosmic threat is approaching Earth.

One evening, the Fantastic Four witness a strange phenomenon over New York City as the sky turns orange and a mysterious silver-skinned woman riding a surfboard descends into Times Square. She introduces herself through a cryptic warning as the herald of Galactus, an immensely powerful cosmic entity that travels across the universe consuming entire planets to sustain himself. Earth is his next target. Johnny attempts to pursue the mysterious Silver Surfer, but she effortlessly escapes him after uttering a foreign proverb that he does not understand. Back at the Baxter Building, the team investigates her previous appearances and discovers a horrifying pattern: every planet she visited on behalf of Galactus was subsequently destroyed. Johnny studies recordings of her earlier encounters and realizes that the same unfamiliar language appears in all of them, suggesting that the Silver Surfer may have a deeper connection to the worlds Galactus has consumed.

Determined to confront the threat before it reaches Earth, the Fantastic Four publicly announce their mission and depart aboard the Excelsior with H.E.R.B.I.E. During their journey, they travel faster than light and eventually arrive at a planet being consumed by Galactus. As the world collapses into Galactus’ enormous spacecraft, the team enters the vessel and confronts both the cosmic entity and his herald. The Silver Surfer explains to Johnny that the strange proverb she previously gave him was actually a blessing meaning “Die with yours.” Galactus then reveals the true reason he has come to Earth. He has sensed the extraordinary power contained within Reed and Sue’s unborn child and demands that the baby be surrendered to him. According to Galactus, the child possesses limitless cosmic potential and could inherit the eternal hunger that drives him. The Fantastic Four refuse to sacrifice their child, even if protecting him means risking the destruction of their entire planet.

The team escapes Galactus’ ship, but the Silver Surfer pursues them through a wormhole. Reed devises a dangerous plan involving a neutron star. He intends to trap the Silver Surfer beyond the star’s event horizon while using the neutron star’s immense gravitational force to propel the Excelsior to safety. The plan succeeds, but Galactus’ influence triggers Sue’s labor during the escape. With the team trapped in the middle of a cosmic crisis, Sue gives birth aboard the Excelsior to Franklin Richards. The Fantastic Four eventually make it back to Earth, but their victory is overshadowed by the knowledge that Galactus remains determined to claim Franklin.

Reed publicly announces that Galactus demanded his son and that the Fantastic Four refused to surrender him, regardless of the consequences. As fear spreads throughout the world, public opinion turns against the team. Protesters argue that sacrificing one child is preferable to allowing billions of people to die. Reed struggles to find a solution and becomes obsessed with applying the principle behind Archimedes’ law of the lever: a small, strategically applied force can move something vastly larger. Johnny continues studying the Silver Surfer’s transmissions in search of the metaphorical “lever” that might allow humanity to defeat Galactus. Reed’s determination to anticipate every possible threat also creates tension between him and Sue, who becomes frustrated by his willingness to contemplate catastrophic possibilities. She reminds him that his habit of preparing for the worst can also hurt the people closest to him.

As the crisis deepens, Sue confronts the protesters outside the Baxter Building and recalls how her father fought to keep their family together after a devastating automobile accident. Drawing strength from her parents’ example and her late mother’s determination, Sue declares that the Fantastic Four will do whatever is necessary to protect both Franklin and humanity. Her words inspire Reed to reconsider his research. He realizes that the Teleportation Bridge, which was designed to transport objects across space, might be powerful enough to transport something much larger: Earth itself. The Future Foundation mobilizes scientists and governments around the world to construct a synchronized network of enormous teleportation bridges capable of moving the entire planet out of Galactus’ path.

The plan initially appears promising, but Galactus’ arrival creates an immediate obstacle. The world begins conserving energy to power the massive teleportation network, while the Fantastic Four prepare for the cosmic entity’s arrival. However, the Silver Surfer destroys the bridges before they can be activated. Johnny, who has finally deciphered her language, confronts her and learns the tragic truth behind her actions. She was once an astronomer from the peaceful planet Zenn-La who became Galactus’ herald in exchange for saving her world and her daughter. Although she has witnessed countless planets being destroyed, she continues serving Galactus because she believes there is no alternative. Johnny tries to convince her that she does not have to repeat the sacrifice she made for her own world, but she insists that surrendering Franklin is the only way to save Earth.

With the teleportation network destroyed and Galactus closing in, Reed develops one final plan. The remaining functional bridge in Times Square can potentially transport Galactus to the farthest reaches of the universe, but the cosmic entity must first be lured into position. To accomplish this, the Fantastic Four must make Franklin appear to be the bait. At the same time, Harvey Elder, the Mole Man, agrees to help evacuate New York City’s population into the underground realm of Subterranea. Ben says a painful farewell to Rachel Rozman, the schoolteacher with whom he has developed a romantic relationship, knowing that he may not survive the coming battle.

When Galactus arrives, the Fantastic Four launch their plan. They distract him while secretly replacing Franklin’s real baby seat with an empty one. The actual Franklin is placed in H.E.R.B.I.E.’s care and hidden from Galactus. However, Galactus is too powerful to deceive for long. He discovers the ruse, tracks Franklin to the Baxter Building, and takes the infant for himself. The Fantastic Four desperately pursue him as the battle escalates. Reed, Johnny, and Ben fight to slow Galactus while Sue uses every ounce of her power to force the gigantic cosmic being toward the teleportation bridge.

The struggle pushes the team to its absolute limits. Johnny nearly sacrifices himself to help stop Galactus, but the Silver Surfer finally turns against her master. In a last-second act of defiance, she attacks Galactus and propels him toward the functioning teleportation bridge. The two cosmic beings are transported away together, removing Galactus from Earth and saving the planet. The victory, however, comes at an enormous emotional cost. Sue collapses from exhaustion and appears to have died. Reed desperately attempts to revive her, but nothing works. Just as the Fantastic Four fear they have lost Sue, Franklin instinctively unleashes his extraordinary cosmic abilities and brings his mother back to life.

With Galactus defeated and Earth saved, the Fantastic Four are celebrated once again as heroes. They return to their lives at the Baxter Building and prepare to make another appearance on Ted Gilbert’s television program. Before they can complete the appearance, however, they receive another distress call and immediately leave to help those in need, reaffirming that being heroes is inseparable from being a family.

Four years later, Reed and Sue are raising Franklin in the Baxter Building. One evening, Sue leaves the room briefly to retrieve another book while Franklin remains on the couch. When she returns, she discovers a mysterious cloaked figure sitting beside her son. The stranger removes his mask, revealing the unmistakable face of Victor von Doom. Franklin looks upon the masked ruler, unaware that his arrival may signal the beginning of a new and potentially dangerous chapter for the Fantastic Four.