First broadcast 10 September 2011.

“The Girl Who Waited”, by writer Tom MacRae and director Nick Hurran, is almost a great episode of Doctor Who. Both idea and execution are, for the most part, marvellous. The emotional stakes are high. If it does not quite reach that vaulted ‘classic’ status, and I for one don’t think it does, it is because its climax does not follow through on the potential of the episode overall. The set-up is marvellous. Its conclusion lacks courage.

The Doctor (Matt Smith) takes Amy (Karen Gillan) and Rory (Arthur Darvill) to the planet Apalapucia, in anticipation of a much-needed holiday. Instead they find the planet deserted, save for an automatic door to a room. Press the green button, as the Doctor and Rory do, and the room is a quiet visiting room for speaking to the sick. Press the red button, as Amy does, and the door leads to a time-shifted quarantine zone where years inside will pass while hours do outside.

Attempts to use the TARDIS to extract Amy initially fail, and leave Rory with a dilemma: there are now two Amys, and he can only rescue one. The Amy who just entered quarantine, or the Amy who has lived decades there on her own.

The episode is a showcase for a marvellous performance by Gillan, playing both the regular Amy Pond and her much older equivalent. She performs the latter thanks to some particularly effective prosthetic makeup, and a carefully developed performance that changes her voice and physicality in subtle but significant ways. Arthur Darvill gets some excellent material to play as well, effectively expressing Rory’s confusion, regret, and sorrow at his wife’s awful predicament.

It is a relatively low-budget episode, and in this case Tom MacRae’s strong script swaps out action and guest characters for an intimate and powerful two-hander for the Ponds. It also builds to a brilliantly bleak climax, which I will now discuess – turn away if you have yet to see the episode but plan to.

Rory is left to choose with Amy to save: he can take the younger version, but it will cause the older, tortured Amy to cease to exist. It is at this high-tension moment that the episode fails, because the older Amy – presented as aggressively pleading for Rory to save her – spontaneously changes her mind and sacrifices herself for her younger version.

It is the ‘safe’ choice. It makes the older Amy more heroic in her final moments, and it saves Rory from making an absolutely terrible choice that would haunt him forever. The problem is that it is also an emotional cop-out. It sets up a moral quandary brilliantly, but lacks the courage to follow through on its implications. There is an outstanding opportunity for character development left on the table, in favour of something neat and episodic.

The finished episode is largely very effective and emotional. The shadow of the braver version hovers enticingly above it: something good that could have been absolutely great.