As Reed Richards and Sue Storm prepare for their wedding, a silver object enters Earth's atmosphere, creating immense craters around the world. General Hager directs Reed to track and identify its movements.
During the wedding, Reed's systems detect the phenomenon approaching New York City, which suffers a blackout from the object's electromagnetic pulses. Johnny Storm pursues the object, discovering it to be a silver humanoid on a flying surfboard. The "Silver Surfer" drags him into the upper atmosphere, suffocates and then drops him. Johnny barely survives, successfully flying only at the last moment. Later, Sue and Johnny find they have switched powers after touching, prompting Reed to deduce that exposure to the Surfer has affected Johnny's molecular structure, allowing these transfers of their powers. Tracing the Surfer's cosmic energy, Reed discovers that a series of planets the has visited are all destroyed.
Reed and Sue contemplate abandoning their lives as superheroes in order to have a normal life and raise a family, but are unaware Johnny is listening. With the Surfer creating craters around the globe, Reed determines that the next will appear in London. The team arrives too late to stop the crater, which drains the River Thames, but they manage to prevent the London Eye from collapsing after it is damaged by the quake (although Johnny nearly thwarts their attempt unintentionally when he switches powers with Reed by accident). Meanwhile, the Surfer's movements around the globe bring him past Latveria, where the cosmic energy affects their ex-friend Victor Von Doom, freeing him from two years encased in metal. A scarred Doom traces the Surfer to the Russell Glacier and offers to join forces. When the Surfer rebuffs him, Doom attacks. The Surfer retaliates, blasting Doom through the ice, but the cosmic energy of the Surfer's blast heals Doom's body.
Doom leverages his experience into a deal with the American military, which forces the Fantastic Four to work with Doom. Deducing that the Surfer's board is the source of his power, Reed develops a tachyon pulse generator that will separate the Surfer from it, while Doom devises a machine whose function he keeps secret. In the Black Forest, the Surfer confronts Sue and reveals he is merely a servant to the destroyer of worlds, and regrets the destruction he causes. The military opens fire on the Surfer, which distracts him and allows the Fantastic Four to fire the pulse, separating the Surfer from his board. The military imprisons the Surfer in Siberia, where they torture him for information. Sue uses her powers to sneak into his cell, where he informs her that his master, known by the people of his world as Galactus, is a massive cloud-like cosmic entity that feeds on life-bearing planets to survive, and that his board is a homing beacon summoning Galactus to the planet. The Silver Surfer has to serve Galactus, who will otherwise destroy the Surfer's planet.
Using the device he created earlier, Doom betrays Hager and steals the board from the compound, killing most of the Army personnel there, including Hager. The Fantastic Four rescue the Surfer and pursue Doom in the Fantasticar, confronting him in Shanghai. During the battle, Sue is severely wounded. With the Surfer powerless, Johnny absorbs the combined powers of the entire team in order to battle the now cosmic energy-empowered Doom. Johnny succeeds in breaking Doom's control over the Surfer's board, and Ben Grimm uses a nearby crane to knock Doom into the harbor. However, Galactus arrives and Sue dies in Reed's arms. The Surfer regains control of his board, restoring his power. He revives Sue and chooses to defend Earth, flying, with an extra boost from Johnny, into Galactus. The conflict results in a massive blast of energy that engulfs Galactus, and apparently kills the Surfer as well. Johnny recovers his stable molecular state after touching the Surfer's board.
Reed and Sue marry in Japan, in an abbreviated ceremony. Receiving news that Venice is sinking into the Adriatic, the team heads to Italy. In a mid-credits scene, the Silver Surfer's seemingly lifeless body floats through space. His eyes then open and his board races toward him, hinting that he is still alive.