Easter Island, 1842. Horace Stockwood, eminent archaeologist, has stolen a stone tablet sacred to the islanders. He escapes into the open sea, but massive, sinister stone figures are lining the cliff tops, watching him go...

Thirty years later, Stockwood is desperate to return. He has devoted his life to studying the sacred stone, and needs to know if his theories are correct. Visiting Earth with Leela, the Doctor's interest is piqued, and he offers to fund Stockwood's expedition. But their journey proves more hazardous than anyone would have expected.

What is the terrible secret that pushes Stockwood on - and what is his real agenda? Who is trying to stop their mission before it has even begun? As the Doctor begins to piece the answers together, it seems he may become an accomplice to the terrible tragedy that threatens to befall the island. Ancient powers are invoked, and dangerous secrets may soon be secret no longer...