First broadcast 29 May 2010.

Amy Pond (Karen Gillan) has been dragged far underground into a sleeping hive of Silurians – the original sentient inhabitants of the Earth. While a Silurian held hostage by a frightened group of humans on the surface, the Doctor (Matt Smith) and Rory (Arthur Darvill) race to prevent an inter-species war.

For the veteran Doctor Who fan there is more than a strong sense of deja vu about “Cold Blood”, since it and the preceding episode “The Hungry Earth” largely copy Malcolm Hulke’s 1970 serial “Doctor Who and the Silurians”. The similarities are more overt here, and the storyline arcs up and the Doctor finds himself between two rival species preparing to wipe one another out. Back in 1970 events end in a fairly catastrophic manner. Things are much more optimistic here: a sign that over the intervening decades Doctor Who really has changed quite a bit. That said, Chris Chibnall writes a somewhat open-ended conclusion that begs for a follow-up. To date, Moffat’s 19th century Lady Vastra aside, the Silurians really have not seen much action since. It’s a shame.

“The Hungry Earth” attempted to stage an alien incursion with effectively one alien: Neve McIntosh’s warrior Alaya. Budget constraints mean that “Cold Blood” does not do too much better, effectively giving the viewer three more named Silurians plus a bunch of masked and silent warriors to stand around in the background as extras. What is more, one of the three new speaking roles in Alaya’s twin sister Restac – meaning Neve McIntosh is playing a double role. It is a shame that the superb original design of the Silurians was abandoned in favour of a more human-like prosthetic, but with the more limited budget of Season 5 a more faithful adaptation was never going to be possible.

There is running around, and quite a bit of back and forth. Up on the surface Ambrose (Nia Roberts), driven by fears for her husband and son’s lives, winds up accidentally murdering Alaya while attempting to torture her. This act winds up forming the core of the episode: making a terrible mistake and having to live with the consequences. While Ambrose does get her family back, the episodes to its credit does not shy from making her acknowledge her appalling crime.

“Cold Blood” is solidly written but unexceptional. It is only the episode’s final minutes that really jump off the screen. On the run from the Silurian hive, the Doctor, Amy and Rory encounter yet another smiling crack in time – the season’s ongoing mystery. This time around the Doctor actually reaches inside and pulls something out – a fragment from the explosion that seems to have caused the cracks across the universe. It’s a piece of the TARDIS, in what is a genuinely surprising and ominous development. What is worse Rory is shot by a dying Restac: when his dying body is hit by the light from the crack, like the soldiers in “Flesh and Stone” he is erased from history. Amy’s trauma at his death and his imminent erasure is powerfully performed by Karen Gillan – it is the best performance she has given in the series so far. Then she forgets him, in an appalling moment, and only the Doctor and the viewer are left to know what a terrible, unimaginable horror has just occurred. It is tremendously creepy. Kudos to Arthur Darvill, whose strong performance over the past few episodes really does make his death hit hard.

Quite why the Doctor can reach into the crack when others even getting close to it cease to exist is not explained, and it actually forms part of what seems like a growing pattern: as showrunner Steven Moffat is very good at cool ideas and iconic moments but he shows a sort of hand-waving disregard for internal logic and continuity. He could provide a line to explain how the Doctor can touch the crack, but he simply does not seem to care all that much about the fine details. It all works on an emotional level at the time, and the shock of finding a TARDIS fragment in the crack is jaw-droppingly effective, but as his tenure as producer goes on and the story arc develops from Season 5 to Season 6 the vagueness does become quite maddening.