Indy is in a race against the Soviets to return the Crystal Skull to its rightful place.
Marion (Karen Allen) returns, revealing that Mutt (Shia LaBeouf) is Indy's son.
Mac (Ray Winstone) is a double-agent, working for the Soviets.
Indy, Marion, Mutt, and Oxley (John Hurt) get out of danger.
The crystal skull is really the head of an alien, which came to Earth thousands of years ago and once the skull is returned the Kingdom is destroyed and the aliens leave in a UFO.
Marion and Indy get married in the end.
NOTE: Herbmeistr reports that the crystal skull belongs not to an alien, but an interdimensional being.