The writer confirmed to Doctor Who Magazine that the instalment will reference elements of the show's past.
"It all starts in void-space, with something or someone we have not seen since [1969 story] 'The War Games', and a knock on the Tardis door," he explained. "The guest star... is the brilliant Suranne Jones, playing someone who is beautiful... and who bites. [She] might just turn out to be an old acquaintance with a new face."
Gaiman's episode is expected to air third in the next run, but he explained that the story was originally intended to form part of Matt Smith's first series as the Doctor.
"Steven Moffat wrote me the letter telling me that they were out of money, and that because I'd written a very expensive episode they were bouncing me to episode three of the new season," he claimed.
He added that the move required rewrites to incorporate the character of Rory (Arthur Darvill).
"I wrote another version of it, with Rory in, because now he existed again," he quipped.
Series star Matt Smith recently told DS that Gaiman's episode is "brilliant" and "a real one for the ardent Who fans".
Doctor Who will return to BBC One for a full series in 2011, following a one-off special this Christmas.