The full story
Miles Morales is a Brooklyn teenager caught between his police-officer father Jefferson Davis, who distrusts Spider-Man, and his uncle Aaron, who takes him out to paint graffiti. In an abandoned subway station, a radioactive spider bites Miles, and he starts developing powers he doesn't understand.
The crime boss Wilson Fisk — this world's Kingpin — has built a particle collider under the city, trying to pull in alternate versions of his dead wife and son from other universes, having blamed this world's Spider-Man for their deaths. When Miles stumbles onto the machine, he watches Peter Parker die fighting Fisk's enforcers, handed a flash drive meant to shut the collider down for good with his last words.
The collider's activation also pulls in five other Spider-People: a jaded, middle-aged Peter B. Parker from another dimension, wrecked by a failed marriage and ready to give up; Gwen Stacy, a Spider-Woman from a world where she got the bite instead of Peter; the noir-styled Spider-Man Noir; Peni Parker, who pilots a psychically-linked mech suit; and Spider-Ham, a cartoon pig version of the hero. All of them are dying by inches the longer they stay outside their own universe.
Peter B. reluctantly starts training Miles, planning to fix the collider and go home himself rather than face his life. Miles keeps failing to control his new powers under pressure, and Peter's own guilt and grief keep surfacing. It's only after Jefferson, not knowing Miles is Spider-Man, tells his son he believes in him anyway, that Miles finally makes his own suit and commits to the fight — and Peter starts pulling himself back together too.
In the final battle, Miles throws Fisk directly into the collider's kill switch, destroying the machine. With it gone, Miles uses the flash drive to send the other five Spider-People home, says goodbye to Peter B., and steps into being Brooklyn's own Spider-Man. In the mid-credits scene, Gwen finds a way to reach across dimensions to Miles again; in the post-credits scene, Miguel O'Hara arrives in the universe of the 1967 Spider-Man cartoon and squares off against that world's Spider-Man.