Original: Following the resignation of Gordon Brown as Prime Minister and Labor Leader, Harman became Party Leader and replaced Conservative Party Leader David Cameron (who became Prime Minister) as Leader of the Opposition. She served in both roles until the Labor party elected Ed Miliband as Leader.
Simplified: Following Brown's resignation as Labor leader on 10 May 2010, Harman was the acting leader of the Labor Party until Ed Miliband was elected on September 25, 2010.