The TARDIS lands on a remote island, surrounded by a sea of acid. They meet Arbitan, keeper of the Conscience of Marinus who forces the travellers to go in search of five keys.
| Name | Type | Role | |
|---|---|---|---|
| Terry Nation | Writer | ||
| Peter Stenson | Guest Star | ||
| George Coulouris | Guest Star | ||
| Gordon Wales | Guest Star | ||
| John Gorrie | Director |