Line of Duty series 1-3 recap: everything you need to know before the BBC drama returns

Get up to speed on the first three series of the BBC's thrilling cop series ahead of the new episodes

What happened in Line of Duty series two?

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The action began with an armed attack on a police convoy transporting a protected witness. DI Lindsay Denton (Keeley Hawes) was the only survivor, and was immediately suspected of being a criminal insider.

Denton proved a formidable case for AC-12, outwitting them at almost every turn and earning Keeley Hawes rave reviews for her performance.

The protected witness turned out to be none other than Tommy Hunter from series one. In fact, it was Dot who masterminded the hit on the police convoy, ordering the killing of Hunter in order to protect his gang paymasters and his own true identity.

Denton was an accomplice, convinced to help after watching a teenager being abused by Hunter. However, she never knew the full picture or that Dot was the man behind it all; she was left alive so that she would be blamed for the attack.

And blamed she was: at the end of the series Denton was convicted of conspiracy to murder. Dot, who had been so ‘helpful’ with AC-12’s inquiry, became a permanent member of the team, tasked with discovering the identity of ‘The Caddy’ after the anti-corruption unit got wind of the codename.

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Unsurprisingly, that wasn’t a case Dot was particularly keen on solving. The corrupt copper was now at the heart of the force’s anti-corruption team. A plum posting indeed – and one which would have serious repercussions in series three…