Set just six short years after the events of Back to the Future III, Marty McFly and The Doc once again embark on a time-travelling adventure!
It's the year 1991, and Marty is still hanging around Doc's house in Hill Valley, now joined by Clara Clayton, and their two sons Jules and Verne. Marty's in junior college and is still trying to put together a rock band, but makes time for an adventure with his old pal Doc Brown.
Marty is using his new keyring to play cat noises, part of a practical joke on the Brown family dog, Einstein. He is quickly attacked by the mutt and chased up a tree. Jules comes to Marty's rescue and they tell Doc about Einstein.
After the dog interferes with Doc's latest invention, a housecleaning robot, the family realise that Einstein could be ill. It turns out to be cat-aracts (a disease which makes the dog view everything as if it were a cat).
With the disease so rare that a medicine hasn't been made in years, Doc uncovers the DeLorean (despite the fact it was trashed in the third movie by a freight train) and goes back to the Prohibition era in 1920s along with Marty, in order to find a distiller where they can brew up a medicine and save Einstein from blindness!