From my biography, extended a bit for more detail. I'm not expecting all of this to be in a summary, but this could give you all an idea of Dent's key moments in Season II:
Harvey Dent grew up in Gotham City, raised by his abusive, alcoholic father. As soon as he was old enough, he escaped and left for law school. Becoming a successful attorney, Dent earned a job in the District Attorney's office in New York City, where he tried unsuccessfully to build a case against Wilson Fisk, AKA The Kingpin of Crime.
He was only supposed to be visiting Gotham, attending a fundraising gala. But the murder of Carl Finch at the hands of The Joker left Gotham without a District Attorney, and Dent agreed to temporarily fill the role. But once in office, he realised that he had a chance to finally make a difference, and do some good. He soon declared war on crime in Gotham - focusing on powerful gangsters Carmine "The Roman" Falcone and Sal "Boss" Maroni - and enlisted the aide of Commissioner Gordon and the mysterious Batman as his allies.
But since starting his mission, Dent has been faced with serious tests of his commitment. His wife, Gilda, was kidnapped by Scarecrow and Electro, in a plan orchestrated by Falcone and Maroni. Close to backing down, it took the support of Batman for Dent to find his resolve, and stand up to his tormentors. With the help of Batman, Dent was reunited with Gilda, and Scarecrow and Electro were apprehended.
He was only supposed to be visiting Gotham....but hasn't left since his return. Perhaps this is where he's meant to be. Harvey Dent has been newly appointed as Gotham's permanent District Attorney. But during his acceptance speech, in front of a crowd of cheering Gothamites, Harvey and Gilda were gunned down in an assassination attempt. Their fate now hangs in the balance...