The earliest poems in the collection do not imply a close personal relationship, instead they recommend the benefits of marriage and children. With the famous sonnet 18 ("Shall I compare thee to a summer's day") the tone changes dramatically towards romantic intimacy. Sonnet 20 explicitly laments that the young man is not a woman. Most of the subsequent sonnets describe the ups and downs of the relationship, culminating in an affair between the poet and the Dark Lady. This seems to end when the Fair Lord himself succumbs to the Lady's charms.
There have been many attempts to identify the Fair Lord. Shakespeare's patron the Earl of Southampton is the most common candidate.