Ewan Mcgregor as Reed Richards/Mr Fantastic
Charlize Theron as Susan Storm/Invisible Girl
Kevin Dillon as Benjamin Grimm/the Thing
Garret Hedlund as Johny Storm/Human Torch
Sam Witwer as Namor, the Sub-Mariner
Clive Owen as Victor Von Doom/Dr Doom
Jackie Earle Haley as Mole Man
Dianna Agron as Melissa Highster
The film begins exactly one year after the four joined together and defeated Dr Doom. They have faced the Mole Man since then, with the film starting with the Fantastic Four battling the Mole Man and an army of underground monsters. The Fantastic Four defeat them all when the Thing and the Invisible Girl bring down his entire underground lair. Once they're back on the surface, Johny goes to a bar. There, he meets someone who says he knows the Sub-Mariner. Johny goes up to the man, and sees he has the strength of Namor. Johny realizes he is Namor, so he chases after him, trying to remind him he is Namor. One thing he does is shave off the man's beard. Johny finally gives him his memory back when he drops the man in the ocean. There, he regains his memory, and realizes he is Namor, the Sub-Mariner. He searches for his former kingdom of Atlantis, but finds it in ruins. He returns to the surface and swears revenge on the surface world for destroying Atlantis. He attacks Johny Storm, which puts him in the hospital. There, she meets a nurse named Melissa Highster. They begin to date, while the remaining Fantastic Four members fight Namor. After defeating them, Namor returns to the bottom of the ocean. There, he finds an underground lair, with Dr Doom inside. Doom makes an agreement with Namor to attack the surface and the Fantastic Four. Namor gathers some underwater creatures to attack the surface. Meanwhile, Doom is attacking three of the Fantastic Four at the Baxter Building. The missing member was Johny, who was still at the hospital. Dr Doom reveals to the Fantastic Four that he survived the exploion a year ago by going through a secret escape tunnel that led him to an underground lair he had set up. Dr Doom brings down the Baxter Building, seemingly killing the three. Johny sees a News report of this on TV, along with Namor's attack on the surface. He gets out of the hospital bed, and flies out on the seen. He first encounters Namor, whom pounds on him until Doom comes in. Doom is just about to kill Johny, until the Thing comes. He fights Doom, and reveals that he was just able to keep the Baxter Building from falling on them. The Invisible Girl shows up, and Doom attacks her. Namor becomes furious over this, and attacks Doom. However, the Thing attacks Namor, while the Human Torch and the Invisible Girl circle Doom. They fight, with Doom and Namor gaining the advantage. Then, Mr Fantastic shows up, but not alone. He has an army of mind controlled monster's of the Mole Man's with him to fight Namor's creatures. They eventually over power them, and Namor's creatures are driven back into the ocean. That leaves Namor and Dr Doom against the Fantastic Four. After a long battle, the FF drive Namor back to Atlantis, and Doom is taken into custody.
In a post credits scene, Melissa is on a date with Johny. When she goes to the bathroom, she takes a device and puts it on her wrist, and says "The Infiltration has begun." She then shapeshifts into a Skrull.
Originally, there was suppose to be a spin-off/prequel titled Namor: the Sub-Mariner. A cast was signed on, and a script was written. However, for unknown reasons, the movie was cancelled.
A sequel, titled "Fantastic Four: Reign of the Skrulls" has been confirmed.