Indy is experiencing/somewhat at odds with a domesticated life with Marion. He still teaches, still does digs, but no gunfights or fistfights. His relationship with his son hasn't quite gone smoothly either.
Then Indy gets word that the Russians are very close to discovering a certain ancient device in the Carpathian mountains. Indy knows this device is very dangerous, but knows too that he has a different life now. Until...
...he learns that his own son Mutt has gotten involved with a nasty group of baddies. He owes them a big debt and agrees to help them track this device down, knowing not only his debt would be paid, but oh the fortune and glory!
Indy tries to track Mutt down, to talk sense into him, but he can't. Indy knows that his son is in over his head with these baddies, but finding this device will be more dangerous yet.
Now Indy must race against his own son to obtain the device before he does. With the help of an old friend on an adventure that reaches from the Gobi Desert to the catacombs of Romania, Indy will reconnect with his son, face old demons, and again be that determined thorn in the side of the Russians.
Oh yeah and Lawrence Kasdan writes it.