Peter had no family, and lived by himself in a house in London. One day at home, he tripped and fell down the stairs, mortally injuring himself. He was saved by the Trickster, who he believed to be an angel because the Trickster had appeared before him dressed in white, not his usual black garb. Presumably at the Trickster's request, Peter seduced Sarah Jane Smith in 2009.
They met at a shop and he gave her his phone number. Sarah Jane didn't use it, but when they met again she thought that for once she could live a normal life. They began dating, though she didn't tell her family and friends. They soon found out and he met Sarah Jane's son, Luke, and they got along well. He proposed while on a date to Sarah Jane and their wedding was set for two weeks later. The Tenth Doctor knew about the Trickster's evil plot and turned up before they could marry, but so did the Trickster.
The truth about Peter's accident was later revealed. Before he died, the Trickster had got his agreement. Peter and Sarah Jane had legitimately fallen in love and Peter said it was Sarah Jane's love that gave him the strength to reject the Trickster's bargain, banishing him again. Peter sacrificed his life for the love of his life and disappeared forever. (TV: The Wedding of Sarah Jane Smith)