Elm Tree Terrace, a London boarding house, is host to an odd collection of guests. Could it be the scene for mystery, or a nascent love affair? After a chance meeting with a stranger at Australia House, Colin Jepson a young graduate from Sydney turns up unannounced to look for a room. Mary Mercy the young woman who checks his references puzzles Colin: she is not English, but perhaps European, yet her dress belies this. Colin is entranced by her eyes, which bear the colour of a lion’s. The other guests at the boarding house fit their stereotypes: a middle-aged school teacher from Scotland and a couple visiting from Adelaide. The boarding house manager, Mr Suddern, seems out of place, perhaps an ex Army type with a penchant for alcohol. Huston, his business partner, is suave with an air of mystery. He has connections to Africa. Colin takes a bus ride with Mary, ending up at London zoo. She tells him of her youth growing up in the Belgian Congo, and of her strange relationship with Huston. The happiness of her ailing grandfather, confined to his room at Elm Tree Terrace, is important to her. Could there be blackmail involved? Colin and the man from Adelaide, with very different objectives, hatch a plan together. When Huston demands Colin give up his room, and an eminent physician comes to dinner the truth behind the Adelaide man and Huston is revealed.