The book overall contains less detail concerning specific events than a typical autobiography. This is because Lewis' purpose in writing about his life was not primarily for historical purposes; instead, his aim was to identify and describe the events surrounding his accidental discovery of and consequent search for the phenomenon he labelled "Joy". This word was the best translation he could make of the German idea of Sehnsucht, or longing. That is not to say that the book is devoid of information about his life. Lewis recounts and remembers his early years with a measure of amusement sometimes mixed with pain. However, while he does describe his life, the principal theme of the book is "Joy" as he defined it for his own purpose.
This Joy was a longing so intense for something so good and so high up it could not be explained with words. He is struck with "stabs of joy" throughout his life. He finally finds what it is for, at the end.
He also talks about his experiences at Malvern College in 1913 at the age of fifteen. Though he described the school as "a very furnace of impure loves" he defended the practice as being "the only chink left through which something spontaneous and uncalculating could creep in".
The book's last two chapters cover the end of his search as he makes the leap from atheism to theism and then from theism to Christianity. Lewis ultimately discovers the true nature and purpose of Joy and its place in his own life.
The book has no connection with Lewis' unexpected marriage in later life to Joy Gresham. This marriage occurred long after the period described, though not long after the book was published. Lewis' friends and contemporaries were not slow to notice the coincidence, frequently remarking that Lewis had really been "Surprised by Joy".
Surprised by Joy is also an allusion to William Wordsworth's poem, "Surprised By Joy--Impatient As The Wind", relating an incident when Wordsworth forgot the death of his beloved daughter: