Jack and Sarah is a British romantic comedy about Jack and Sarah, a married couple who are expecting a baby together. The plot makes an unexpected as a complication arises during the birth of the child, which eventually leads to Sarah's death. With his wife's gone, Jack is so grief stricken, and unable to bear the stress of losing Sarah, that he goes on an alcoholic bender. During this time, Jack leaves his daughter in the arms of his own parents, as well as Sarah's mother. Jack's parents and realize that they need to take drastic action, and so, they decide to return the baby to Jack, without him knowing, while he is asleep.
Jack, now realizing that he has to take responsibility in caring for his child, begins to do so, although he initially struggles with it at first. He eventually begins to get the hang of it and decides to name the child Sarah. Despite getting the hang of it, he finds it extremely difficult to bring up a child with his high demanding job. Even with the help of both sets of grandparents, as well as a washed out former alcoholic, Jack still needs more help.
Jack eventually meets Amy, a beautiful young waitress he meets in a restaurant. Amy immediately takes a high liking to Sarah, and offers to become her nanny full-time. Amy decides to move in with Jack after only one meeting.
During the film we see a major clash of personalities between all of the characters, particularly between Jack's mother, Margaret and William, the ex-alcoholic. As Jack slowly begins adapting to the ever fluctuating demands in his life, the plot thickens as we learn Jack's boss has taken an interest in him as well.