Parts: 6
Writers: Mervyn Haisman, Henry Lincoln
Airdates: 3 February 1968 - 9 March 1968
Production Code: QQ

Doctor: second
Companions: Jamie, Victoria

Creature Feature: Yeti

Story: Professor Travers, now in his sixties, tries in vain to reclaim a Yeti he brought back from Tibet and sold to a local museum. But the owner refuses to sell, despite Travers' protests that the Yeti could become dangerous - he has managed to reactivate a control sphere, but it has subsequently disappeared. His daughter Anne persuades him to go back and search the laboratory once more - but just after they leave, the control sphere arrives at the museum and enters the Yeti, which comes to life and kills the museum owner.

Some time later, the TARDIS is arrested in space by a pulsating white web which begins to form around it. When the craft breaks free, it materializes in the London Underground. The Doctor, Jamie and Victoria discover that London has been evacuated, and people have been killed, their bodies covered in a weblike substance. They then find soldiers laying down cable in the tunnel; while Jamie and Victoria follow them, the Doctor traces the cable to its endpoint...and finds Yeti, armed with web-guns, roaming loose in the Underground.

The military have established a base in the Underground, from which they hope to find a way to combat the Yeti threat. But the surface is swathed in a thick mist from which people who enter never return, and the tunnels are slowly being engulfed by a fungus that threatens to hem them in. Even with the help of the Doctor, Jamie, Victoria, Professor Travers and Anne, it seems impossible to combat the threat presented by the indestructible Yeti...and no-one even knows yet what the ultimate plan of the Great Intelligence is.