Henry Quantum has several thoughts going through his head at any given time, so it's no surprise when he forgets something very important--specifically, a Christmas gift for his wife, Margaret, which he realizes on the morning of December 23. Henry sets off that day in search of the perfect present for Margaret: a bottle of Chanel No. 5. But much like Henry's ever-wandering mind, his quest takes him in different and unexpected directions, including running into the former love of his life, Daisy. Margaret, meanwhile, has begun her own affair, seeking an escape from her unhappy marriage. And Daisy, who has made the unsettling choice of leaving her husband to strike out on her own, finds herself questioning whether she and Henry belong together after all.