Terrorists have planted a deadly virus on a transcontinental train. On board are the glamorous Jennifer Chamberlain (Sophia Loren) and her ex-husband, Dr. Jonathan Chamberlain (Richard Harris), as well as Nicole Dressler (Ava Gardner) and her young lover, Robby (Martin Sheen). They can't get off, and one solution proposed is to quarantine them by rerouting the train over the Cassandra Crossing -- an unstable bridge. Dr. Chamberlain wants to save them, but has to battle the terrorists first.