Slow Horses returns for a sixth season with the same mix of sharp humour, messy espionage and spectacularly flawed intelligence operatives that has made the Apple TV series a standout spy drama.

Gary Oldman is back as the foul-mouthed and unpredictable Jackson Lamb, while Jack Lowden returns as River Cartwright. This time, however, the danger surrounding Slough House appears to come from outside rather than from the team’s usual catalogue of mistakes.

The season opens with the death of David Cartwright and River preparing for his funeral. Soon after, former Slow Horses begin turning up dead. The first victim is Struan Loy, the hapless operative Lamb dismissed back in the first season.

River and his colleagues discover that the killings are connected. Two assassins are systematically targeting former members of Slough House, while MI5 has apparently erased the victims — and Slough House itself — from its official records.

That premise gives Season 6 a darker edge without sacrificing the show’s trademark wit. The contrast between Lamb’s cynical leadership, River’s determination and the wider incompetence of British intelligence remains central to the series’ appeal.

Once again, Slow Horses turns bureaucratic failure, professional rivalry and espionage into hugely entertaining television, proving that its formula still has plenty of life left.