The Doctor and the Brigadier begin to realise who the Zygon imposters among them are while Sarah Jane discovers a secret tunnel to the underwater ship.
| Name | Type | Role | |
|---|---|---|---|
| Robert Banks Stewart | Writer | ||
| Lillias Walker | Guest Star | ||
| Ronald Gough | Guest Star | ||
| Peter Symonds | Guest Star | ||
| Douglas Camfield | Director |