The TARDIS is engulfed by a mysterious web-like substance in space. Breaking free, the Doctor and his companions Jamie and Victoria land in the London Underground system. But a spreading killer mist has meant that London has been evacuated. The Great Intelligence is back – and its robot Yeti are roaming the streets and the underground tunnels. The Doctor, Jamie, and Victoria team up with an army unit working with their old friend Professor Travers to combat the menace. They soon realise, however, that someone in the group must be working for the Intelligence. With time running out, the Doctor and his friends realise they can trust no one…