In a parallel England, the Doctor tries desperately to convince his captors of the immense peril they are in. But they are convinced he is a spy. And penetration zero is getting close…
| Name | Type | Role | |
|---|---|---|---|
| Don Houghton | Writer | ||
| Sheila Dunne | Guest Star | ||
| Nicholas Courtney | Guest Star | ||
| Ian Fairbairn | Guest Star | ||
| Olaf Pooley | Guest Star | ||
| Roy Scammell | Guest Star | ||
| Christopher Benjamin | Guest Star | ||
| John Levene | Guest Star | ||
| Derek Newark | Guest Star | ||
| Douglas Camfield | Director | ||
| Barry Letts | Director |