Drawn into the orbit of the planet Vortis, the Doctor and his companions (Barbara, Ian, and Vicki) encounter three sets of non-humanoid inhabitants: the Zarbi, the Menoptra, and the Optera. The Zarbi are under the control of a sinister, spider-like central force, the Animus, which uses gold as a conductor for its mesmerising power. After a disastrous ambush of the spearhead Menoptra force attacking the Animus, Barbara manages to destroy it with an act of concentrated willpower. With the Zarbi freed from the Animus's malevolent control, they, the Menoptra, and the Optera are free to live on Vortis, while the Doctor and his companions leave in the TARDIS.