While it’s easy to over-state the differences between companions (they’re mostly there to ask questions, get into trouble and be the Doctor’s strength and weakness), the way they each fulfil their role can vary a lot. Dropping Sarah Jane into a UNIT story creates a very different dynamic than with Liz or Jo, both UNIT employees. Sarah needs both a reason to be accepted by the Brigadier, and a reason to want to stick around. Last episode dealt with the former essentially by having the Doctor appoint her his temporary assistant. This episode focuses on what’s in it for Sarah. She’s not getting paid to be the Doctor’s assistant, so she needs a different angle: a story, ideally with some juicy photos of a captive tyrannosaurus. Later, she smells something interesting about a secret nuclear reactor, and goes off to interview Sir Charles Grover MP in a very business-like way. The result in both cases is the same as would have happened with Jo: Sarah gets into danger. But the route to the result is quite different, and spices up what could have been fairly routine.

Sarah’s independent streak also means that she’s playing hard to get with the Doctor. Usually in the classic series when a companion is introduced, they’re best buddies with the Doctor by their second story. Here, Sarah’s ability to go off to handle a sub-plot under her own steam is a bit disconcerting for the Doctor. Note the scene where Sarah starts to piece together who on Earth might be behind the Dinosaur invasion: at first, her line of thinking is treated with off-handed indulgence by the Doctor, until she challenges him to take her seriously and comes up with the goods: the elusive Professor Whittaker. Later, the Brigadier and the Doctor joke about her having a ‘beautiful friendship’ with General Finch – only for the Doctor to be very abashed when he learns she has, in fact, gone off in the general’s car. It’s like he’s found out the person he’s started dating isn’t exclusive, and he needs to put a bit more effort in. It’s not hard to see why she becomes such a consequential character.

The cliffhanger emphasises how much Sarah has become a co-lead in just seven episodes. Having pulled at the thread of the conspiracy (not difficult, to be fair, given everyone except the Brigadier and Benton are in on it) and discovered it reaches the very centre of government, Sarah is captured, hypnotised, and wakes up to be welcomed ‘to the people’ by a mysterious young man called Mark, who claims, in a quite stunning twist, that they are on a spaceship three months from Earth, en route to a new world.

Next episode: Invasion of the Dinosaurs – Part Four