The Doctor solves the murder mystery and finds the final missing key. With Ian now free, the travellers return to the island only to discover that Arbitan has been murdered.
| Name | Type | Role | |
|---|---|---|---|
| Terry Nation | Writer | ||
| Donald Pickering | Guest Star | ||
| Alan James | Guest Star | ||
| Fiona Walker | Guest Star | ||
| Katherine Schofield | Guest Star | ||
| Peter Stenson | Guest Star | ||
| Robin Phillips | Guest Star | ||
| Michael Allaby | Guest Star | ||
| Martin Cort | Guest Star | ||
| Henley Thomas | Guest Star | ||
| Stephen Dartnel | Guest Star | ||
| John Gorrie | Director |