Episode 1

An abortion doctor is shot dead in his own clinic, and the killing has an American accent. Mary Kane is a pro-life activist with twenty more bombs to place and a knack for recruitment: she has started training like-minded groups on British soil. What begins as a single murder investigation turns into a national threat, and the episode never lets you forget the size of it.

The informant does the arithmetic for us early on. The doctor was the tip of the iceberg; the rest is still out there. The bombs are out there too, and the British groups she has been training are the part that keeps the team awake at night. Mary Kane is the awkward kind of enemy. She answers to her own convictions and nothing else. And true believers are harder to predict than anyone on the books.

For a series opener it is an unusual choice of villain, and all the more frightening for it. She is one woman, and the hour makes that the point. The informant's word is all the team has to go on, and it is plenty. The hour's early calm is the kind that never lasts, and the waiting is part of the threat. There is no grand conspiracy to explain Mary Kane away, just a woman with a plan and the patience to carry it out.

David Wolstencroft wrote the script, Bharat Nalluri directed. Jenny Agutter's Tessa Phillips and Lisa Faulkner's Helen Flynn do much of the legwork, with Lisa Eichhorn's Kane a cool, patient presence at the centre of it all. Ken Bones, Megan Dodds, Matthew Delamere and the rest of the cast fill out a first episode that is confident enough to take its time.

As opening statements go, it's a strong one. Twenty bombs is a lot of bombs, and the episode keeps count of every one. Later seasons would build bigger conspiracies. This one does the groundwork, and it does it well.