In one timeline, Martha has to choose sides and Tom gets caught in the crossfire. In the other, Catherine and a new friend get saved in the worst possible way.

Molly Windsor as Martha, Tom’s alien girlfriend, gets a lot to do this episode in a way that cleverly mirrors Bill’s experiences last season. Tom’s empathy and care for her is haunting, but what haunts her more is the fact she’s pregnant: the exact thing she’s been told she can’t be and that her race can never carry to term. Windsor does a great job of showing her building hope and determination and the episode neatly explores the effect that has on her comrades. Martha is now everything they’ve convinced themselves they can never get and that terrifies them. The aliens aren’t without potential anymore, they have a future. Maybe. If they’re brave enough to try.

Elsewhere in the episode Tom and Bill get some bonding time after Tom is shot surveiling Martha and that friendship continues to be repaired in a surprising, muted way. There’s also a mounting sense of dislocation as Zoe and her team prowl a London that isn’t quite a battlefield again, but is on the verge of one.

The bulk of the episode’s best work happens in the original timeline though. Catherine, held hostage at first by a kindly and terrified young man called Sam who needs her help transporting his sick parents, has the worst of times and finds out a lot. The black hole is scrambling brain waves and anyone outside for more than 45 minutes sinks into a coma and dies and that’s why she ends up dragging two invalids through the Underground’s tunnels. There’s a moment of neat, black comedy when she asks to stop for a bathroom break and her captor politely asks she not ‘…go…on my mum.’

But the comedy, bleak as it is, is short-lived. When they reach the hospital they’re drugged  and captured. It is a hospital but not for their side. This dovetails smartly with the Martha plot as in both timelines, the aliens need blood transfusions to live.

Verdict: This is a builder of an episode that also makes clear how the treads of the show connect. It’s typically clever, typically bleak and another strong entry. 8/10

Alasdair Stuart