The third Doctor and Jo find themselves on an artificial world inside the black hole while the second Doctor tries to find a way to control the organism.
| Name | Type | Role | |
|---|---|---|---|
| Bob Baker, Dave Martin | Writer | ||
| Roy Purcell | Guest Star | President of the Council | |
| Clyde Pollitt | Guest Star | ||
| Denys Palmer | Guest Star | ||
| Laurie Webb | Guest Star | ||
| Lennie Mayne | Director |