The first half focuses on resolving the water plot from last week. Lucarotti comes up with an ingenious solution: condensation inside the TARDIS, which is also an opportunity for Ian to give Marco another Science 101 lesson. I wonder if this is what Sydney Newman had in mind when he said the series should be educational? Later, there’s a charming scene in which Ping-Cho tells the story of Ala-eddin and the Hashashins, and Ian gets to tell us how the word assassin came about. It’s all very worthy, but quite how any of it advances the story is anyone’s guess. I listened to Ping-Cho’s tale wondering if there’d be some thematic link to Tegana’s plotting, but if there was it escaped me. At moments, Five Hundred Eyes risks being little more than a particularly interactive Coal Hill School class.

Fortunately, when Lucarotti does remember this is meant to be an adventure in space and time and not a lesson in science and history, the episode is quite good. Giving the Doctor access to the TARDIS isn’t just a handy solution to the water shortage, it also means he has chance to palm a spare key, raising the prospect of escape. Having befriended Ian during the adventure of the broken switch, the Doctor’s ire is now pretty much reserved for Marco, and the duplicate key sub-plot adds another dimension to the game of one-upmanship between the two of them.

One curious thing I’ve noticed about these early episodes is how central the TARDIS is. In later seasons it’s pretty much just a mode of transport, but in these first 16 episodes it’s been a home, a quasi-haunted house, a thinking machine, and now the coveted prize in a three-way tug-of-war between the Doctor, Marco and Tegana.

Meanwhile, Tegana’s own power games with Marco continue. The second half of the episode, which gets away from doing Jackanory in tents, is very strong. The Cave of Five Hundred Eyes is a particularly creepy-looking set, with fearsome painted faces and carvings on the walls. Having Barbara, now well established as the most intuitively correct character, suspect Tegana is up to no good is a clever move, because it adds to the tensions in the camp. It also creates some real peril as she trails Tegana to the cave, and is captured, with – in a scene I’m sure wouldn’t be permitted now – a dagger held across her throat. This gives the Doctor his first chance in weeks to do something more than sit, complain or fall over, as he leads a foolhardy rescue attempt accompanied only by Susan and Ping-Cho. The moment when eyes appearing in the blank sockets of a mask, is a nightmarishly horrible image, and the strongest cliffhanger since The Dead Planet’s.

Five Hundred Eyes no longer exists. Not in any of its regenerations. This review is courtesy of the excellent Loose Cannon reconstruction.

Next episode: The Wall of Lies