Your team has successfully located the Lost Ark. Whatever you do, don't look at it! While your team was searching the temple, you found a note. It reads:
I heard about your predicament. Sorry to be the bearer of bad news, but I have taken what was left of the plutonium found in the temple back to Marshall College where I teach as a professor of archaeology. I heard there is a group of men in New York who use plutonium to power special ghost-fighting weapons. I forget what they call themselves...the ghost-biters? Maybe you can ask to borrow some.
Anyways, good luck.
Make your way to the next room and see if you can find the plutonium. Once you get the information you need, meet back up in the windy city. If you can't find the Ghostbusters right away, check some place haunted. They're most likely removing some ghosts, at least, that's what their ad said.