The realistic baddie, the foolish, selfish co-pilot displays the worst in humanity in a believable, relatable way. I love how he leads the drama into the central plot, the prisoners being taken to be sacrificed to a God on another world and a selfish, imperialistic race in awe and frightened of that God who ultimately kills the foolish man. The prisoners, initially barely focused on, give the sequence where the spaceship “crashes” some much needed drama. A superb way to kick off a story.

This might make the story sound very serious - it's not. There is a lot of humour packed into these four episodes. K-9 twisting his neck is particularly hilarious! Lalla and Tom have never been better together. It is great to see Romana given some independence and take charge of the situation on the ship once The Doctor gets stranded. In part four when she verbally takes down the lead human villain she is in fiery form with her superb acting, proving she could easily have taken the lead role in the show herself.

The costume, movement and voice of the Nimons leave a lot to be desired. They are certainly not as well realised as the costumes and props that surround them. It's a shame they didn’t get a killer look for the intelligent beasts, but it's a small quibble which is outshined by the wonderful script, far better than Anthony Read’s previous effort. Concepts like the ever-changing high tech maze walls put a sci-fi spin on these classic themes.

Parts one and two feel particularly strong with it slightly losing its way the further you get through it, especially when it descends into a shoot out. But endings are always trickier than beginnings.

Talking of endings, that marks the final (complete) story from the Graham Williams era. After a dodgy first season I felt the through line of the key to time, then the comedy of season 17 lifted William's performance. It was a good era but one that pales in comparison to what came before it. With Hinchcliffe and Williams' eras considered together, the fourth Doctor's time so far is about as satisfying as the third's.