The Milky Way Galaxy has already started merging with the Andromeda. The planet closest to Earth is a Gladiatorial planet that wants to take over the Earth. The government sends two of the highest trained people it knows to fight this menace.
Ultimate Spider-Man (Miles Morales)
This Spider-Man is the only one who has a Venom Blast, which the citizens of the opposing planet are weak to. He was reluctant at first but he joins when his girlfriend is killed.
The Flash (Barry Allen)
The Scarlet Speedster is able to take out multiple enemies at once. Given powers by a lightning bolt and mixed chemicals, Barry Allen is able to break the speed force and his powers are very valuable to the government.
In the end, Miles and The Flash were surrounded by the remaining survivors of the rival planet. Flash realizes that both of them cannot get out alive. He runs so fast that he breaks the speed force and then pushes Miles' transporter to take him back to Earth. He runs around the planet and drills into the core to make the planet explode. In the end, Barry made the ultimate sacrifice to save the world, along with a 14 year old Spider-Man.