An account of Dr. Robert D. Ballard's exploration of the wreck of the Britannic in September 1995. Britannic, the sister-ship of the Titanic, was sunk after a mysterious explosion while serving as a hospital ship during World War One. Ballard sets out to relocate the wreck and attempts to prove once and for all whether it was a German mine or torpedo which inflicted the fatal damage.

James Cameron brings together some the world's leading Titanic experts, including engineers, naval architects, artists and historians, to solve the lingering mysteries of why and how the 'unsinkable' ship sank.