In view of the growing ecological and economic threat posed by oil spills, this film examines a computer technique to predict the movement of oil slicks, thus enabling emergency clean-up crews to concentrate their efforts in the right place at the right time. As well, such predictions could help in choosing new ports and tanker routes so that damage from an oil spill would be minimal. The probable hour-by-hour movement of both real and hypothetical oil spills is shown on the computer's electronic screen, as it analyses a complex set of tidal, meteorological and oceanographic data.