Doc pulled the suitcase out of the DeLorean time machine upon arriving in 1955 with Marty McFly. It contained various amounts of dollar bills from different eras, allowing him or Marty to purchase items in almost any time period. Bills from 1861, 1863, 1864, 1875, 1902, 1914, 1917, 1921 and 1939 were present in the case.
Doc pulled out a $20 bill that would have been used in the 1950s which allowed Marty to purchase a black trilby hat and black leather jacket as an 'inconspicuous' disguise.
Behind the scenes
- In the novelization, Doc does not have the suitcase but instead wears a money belt which is described as follows:
- " Doc looked about uncertainly. Was there something else? / Doc frowned to himself for an instant, then snapped his fingers and nodded. / 'And you'll need money —' He lifted his Hawaiian shirt to reveal a money belt with close to a dozen different pouches. And each pouch had a label: 1985; 1955; 2015; 1921; gold; silver; doubloons. / 'I have to be prepared for all monetary possibilities,' Doc explained. He reached into the 1955 pouch and pulled out a wad of bills. He handed them to Marty. / 'Get yourself some fifties' clothes,' Doc instructed. 'Something inconspicuous.' "
- —(quote, page 140)
The suitcase was probably used in the film so that all the different currency would be able to be seen on-screen.