Actress Felicity Kendal is to be among the guest stars when Doctor Who returns in 2008, producers have announced.
Best known as Barbara Good in 1970s sitcom The Good Life, Kendal will play Lady Clemency Eddison in the hit science fiction programme.
Filming on the next series has already begun, with comedian Catherine Tate becoming the Doctor's latest companion.
Stars David Tennant and Freema Agyeman both return, and other guests include former Blackadder actor Tim McInnerny.
Over the course of the series, the time-travelling hero will revisit his old enemy The Ood - a race of squid-like humans - and drop in on famous crime novellist Agatha Christie.
"Visiting Agatha Christie has been on my wish-list for ages now, and for the Doctor, it's a real meeting of minds," said executive producer and lead writer Russell T Davies.
Agatha Christie's grandson, Mathew Prichard, said it was a "brilliant idea" to introduce her to the show.
"As far as I know my grandmother never saw Doctor Who, but I am sure she would have been intrigued, excited and above all flattered by all this attention in 2007," he said.
Christie's role will be played by actress Fenella Woolgar, who was recently seen in BBC One's Jekyll.
Catherine Tate said she was "delighted" to be returning to Doctor Who, following her guest appearance as Donna Noble in last year's Christmas special.
She reprises the character in this series - beginning with an episode where she has to track down the Time Lord during an alien emergency in modern-day London.
"I had a blast last Christmas and look forward to travelling again through time and space with that nice man from Gallifrey," said Tate.
Before the new series begins, Doctor Who returns with a Christmas special set on the Titanic and co-starring Kylie Minogue.