A three-man rocket crew are nearly done with their mission to Titan Base until a course change puts the rocket in the path of a strange cloud in space. By the time they arrive, they have come under the control of a sentient virus which threatens the galaxy. When the TARDIS picks up an emergency message, it flies into the cloud, infecting the Doctor. To save himself and others, he must undertake a dangerous journey.
| Directing | Derrick Goodwin | Director |
| Writing | Dave Martin | Writer |
| Writing | Bob Baker | Writer |
| Production | Graham Williams | Producer |
| Writing | Robert Holmes | Script Editor |
| Sound | Dudley Simpson | Original Music Composer |
