The Doctor, Ian and Vicki are captured by the Zarbi while Barbara meets up with the butterfly-like Menoptera who are trying to reclaim the planet.

The Doctor is forced to co-operate with the Animus but Ian uses the opportunity to make an escape attempt.

Ian and Vrestin are captured by the subterranean Optera while Barbara and her Menoptera allies attempt to escape from the slave camp.

Barbara and the surviving Menoptera take refuge in the Temple of Light while the Doctor and Vicki attempt to escape by taking control of a Zarbi.

The Doctor and Vicki are taken before the Animus but the isoptope is left behind in the laboratory.The only hope of victory lies with Barbara and her Menoptera allies.