This novel begins as it ends - as an interior monologue, a soliloquy only the reader hears. Paula Hook, married 25 years to her husband Mike, who is asleep beside her throughout the entire novel, is reflecting on the discussion she and her husband have decided to have with their fraternal twins on their sixteenth birthday (June 10, 1995), which is the "Tomorrow" of the title. Although the book begins with "tomorrow" yet to come, it ends on "today" around dawn. The twins, Nick and Kate, have no idea that this revelation--that they are the products of artifical insemination (AI), i.e., that Mike is not their biological father--is forthcoming.

Paula Campbell and Mike Hook met casually but experienced love at first sight, despite Mike's making the sexual rounds of Paula's friends and roommates. Mike was initially a biologist but becomes a publisher of popular biological publications. Paula is a director in an art gallery in London. Both gradually become more successful and prosperous, have infertility problems and undergo AI. The discussion with the twins never takes place in the novel, which ends at dawn.