Thomas Brewster is haunted by the ghost of his drowned mother. But she is not the only apparition to disturb his dreams. Every few years, he is visited by a mysterious blue box... 

Helped by his new assistant, the young Scots scientist Robert McIntosh, the Doctor struggles to unravel the twisted knot of temporal implausibilities which bind the TARDIS to Thomas Brewster. Meanwhile, lost in the stews of Victorian London, Nyssa must face a host of spectral creatures gathering in the fog.