This Christmas Day, Doctor Who fans are in for an extra special treat, as the BBC will release the fourth and final episode in this series of the Doctor Who: The Adventure Games immediately following the BBC One and BBC HD broadcast of the Christmas special.

The game will be available for UK households on PC and Mac as a free download from the Doctor Who website.

Entitled ‘Shadows of the Vashta Nerada’, this fourth episode sees the first appearance of Steven Moffat’s brilliantly original monsters since their debut in Silence in the Library and Forest of the Dead

Executive produced by Steven Moffat and written by Doctor Who writer Phil Ford (The Waters of Mars, Torchwood), the game sees the return of Matt Smith and Karen Gillan in digital form – as the Doctor and Amy face their biggest adventure yet.

Kicking off immediately following the cliff-hanger ending of episode three, Shadows of the Vashta Nerada sees The Doctor and Amy materialise just south of London (and about a 1000 miles to the left) inside an aquatic sea-bed colony called Poseidon. The game features a breath-taking plot which surrounds a real-world conspiracy – and, of course, takes place at Christmas.

“We’ve gone all-out for the season finale,” says Steven Moffat, executive producer Doctor Who. “Shadows of the Vashta Nerada takes place entirely underwater – something which would be impossible for the TV series, as water is so expensive. It’s thrilling, terrifying, educational and fun. Just steer clear of those shadows…”

Shadows of the Vashta Nerada will be available simultaneously on PC and Mac from the Doctor Who website on Christmas Day. The game will be available for no extra cost to UK households. Users outside the UK will be able to purchase and download it on the same date – details will be put on the Doctor Who website.

Doctor Who: The Adventure Games will return in 2011.