The sun of the gemstone-rich planet Midnight is hostile to all life, but a leisure complex has been constructed there which filters out its deadly radiation. While Donna enjoys some rest and relaxation, the Doctor takes a transport to the famed Sapphire Waterfall. An obstruction on the planned route prompts the transport to take a previously-unexplored path. Suddenly, the vehicle comes to a stop -- and, impossibly, something begins banging on the exterior. As a strange intelligence possesses one of the passengers, the Doctor finds himself fighting a losing battle against the rising tides of panic and paranoia.