Connie James started out at MI5 when she was 19 as a secretary. In the intervening years she became one of the sharpest, most resourceful, no-nonsense operators in the British Security Services. This is because she knew all the secrets (in the course of her duties as an Intelligence Analyst).
When the Ministry of Defence was going to de-commission a nuclear bunker in Surrey containing biological/chemical weapons testing pods; Connie, deciding to retire from MI5, offered herself as housekeeper and lived in the big farmhouse upstairs from the bunker, along with a couple of Labradors, serving as caretaker of the secret bunker. While the bunker remained inactive, it was so top secret only about five people in Britain knew about the bunker. One of them was Harry Pearce, who visited Connie now needing the pods in order to use them. Connie needed a new life; and so rejoined "The Grid" as Ruth Evershed's replacement. Connie was instrumental in assisting the team in her technical role and used her skill to resolve situations.
Connie was later revealed to be a double agent working as a mole for the Russian FSB. The process leading to her exposure began following Lucas North's release and, upon his return to MI5, he revealed the Russians had interrogated him about Operation Sugar Horse; a high-level operation in the 1980's whereby pro-western people in the (then) Soviet Union as sleeper agents, so that they would rise to positions of influence before being activated to find out about Russia's nuclear capabilities. This became the most important operation MI5 had ever undertaken and the most secret ongoing spy operation MI5 ever had.
The team realized that, if the Russians knew about it, then MI5 had to have a very high-level mole. Concerned Sugar Horse had been compromised; Harry sought out and approached his mentor; Bernard Qualtrough a.k.a. "The Spymaster", about Sugar Horse and to help him find the mole. After dismissing the possibility Sir Richard Dolby could have betrayed the operation; Bernard suggested Connie as a likely suspect. Harry arranged for Connie's home to be searched and, eventually after a confrontation with her, discovered a cassette tape hidden in a statue given to her by Hugo Prince; which said Connie knew nothing of the matters that he had discussed with Harry. Harry accepted this as proof of Connie's innocence and was eventually persuaded by Lucas that Bernard was the mole. As it turned out; both Bernard and Connie were working together for the FSB, and the tape was a fake to solidify her innocence.
Connie attempted to frame Harry as the mole with fabricated evidence, but Harry sent Lucas to Moscow to gather intelligence and get to the bottom of his arrest. Lucas returned with a package showing a photo of Connie and Bernard; proving Connie was the mole and not Harry. At the same time, Ben Kaplan had found a form in the paper archive proving Connie was the mole, but Connie herself appeared in the archive to assist Ben in finding the form. Connie saw Ben put the form into his pocket and so, before Ben can leave the archive to warn Ros and the rest of the team, Connie used a bladed garrotte wire, hidden in her bra, and sliced his throat and took the form from his pocket and left, leaving Ben to die. Connie tried to escape, but was arrested by Ros.
Connie was given temporary freedom by Section D in the course of one of their Black Ops; when the team had news that a large scale terror operation was being run by the Russians; codenamed "Operation Tiresias"; which was to launch a nuclear assault on London using portable nuclear devices. Desperate, Connie ended up being the only person who could help the team. The Russians deployed FSB kill teams to hunt the officers down and reclaim Connie, but she was safely ushered through London to the bomb (which Connie had demanded to be brought to). Unable to get the codes in time, Connie began the process of defusing the bomb, removing the radioisotopes; thus preventing a nuclear cascade, before the conventional detonator exploded in her face; the impact of the blast immediately killed Connie.
Before she died, Connie revealed to Lucas North that it was her who betrayed him 8 years previously while he was in Russian custody (though it is unknown if this was true or not; whether or not this was a final act of kindness to help Lucas leave his anger toward Harry behind). In the end, however, Connie died saving her country from the threat of nuclear attack from the Russians.