Plowman married actor Toby Stephens, who is the son of actors Dame Maggie Smith and the late Sir Robert Stephens, in London, in 2001. In May 2007, Plowman and Stephens had their first child, a son named Eli Alistair. Simon Gray, the renowned British Playwright (who penned Japes, a stage play, and Missing Dates, a radio drama, both of which starred Stephens), was Eli's godfather. Their daughters, Tallulah and Kura, were born in May 2009 and in September 2010, respectively.