The Big Four

The threat of world war hangs over this episode, and it changes the feel of everything. Poirot is up against the Big Four, a group of murderous dissidents behind a series of violent crimes. This is not a drawing-room matter. The old company gathers around him anyway: Captain Hastings, Miss Lemon, Assistant Commissioner Japp and George, friends old and new, because the scale of the threat calls for everyone.

The Four are clever enough to stay in the shadows, and Poirot has to work from the edges, from the people who have brushed against them: Tysoe, Ingles, Abe Ryland, Madame Olivier and Stephen Paynter. Any of them could be a pawn, an enemy or a witness. The episode leans into the national anxiety, the sense of a world about to tip over, and the little grey cells get the sort of workout that only an international conspiracy can provide.

Mark Gatiss and Ian Hallard wrote the adaptation with Christie, and Peter Lydon directed. The familiar faces are a large part of the pleasure: Hugh Fraser's Hastings, Pauline Moran's Miss Lemon, Philip Jackson's Japp and David Yelland's George, all back around Suchet's Poirot. Tom Brooke plays Tysoe, Nicholas Day is Ingles, James Carroll Jordan is Abe Ryland, Patricia Hodge is Madame Olivier and Steven Pacey is Stephen Paynter.

The Big Four was always one of Christie's looser books, more thriller than puzzle, and the adaptation gives it a proper cinematic scale. The stakes are national, and for once the case is not about one family's secrets. It is a long way from the usual drawing-rooms. But it works, largely because the threat is allowed to feel real, and because Poirot facing something bigger than a single murderer is a reminder of how far his reputation has travelled.

The finale leans on spectacle and on the assembled cast, and it pays off. Watching the whole team together again, with the world going wrong around them, is exactly the sort of comfort the series does best.