This is generally thought to be the most profound and opaque of all the sonnets. However, if it is thought to be primarily pleading to the young man to lead a better life it might become clearer.

They that have power to hurt, and will do none,

Much has been written about who "they" are. Perhaps it is sensible to accept that the poet has no particular "they" in mind. It could be an aristocrat of great power or any person endowed with natural powers that could be used for good or ill - hence the idea that "they" could hurt you a great deal but because "they" are "good" they do not.

That do not do the thing they most do show,

The "thing" could be anything that a powerful person could obviously do. Again the "thing" would be something wounding. Some universals might be a beautiful person telling you that you are an ugly dork - a powerfully strong man who punches you in the face - a person of high political power who has you arrested without charge - and so on. But the primary concern here is that the boy has the power to hurt the poet a great deal.

Who, moving others, are themselves as stone,

This is a further exploration of the "powerful person's" behaviour. They can affect and influence other people but remain stable and unaffected themselves. Perhaps stone does not mean 'unfeeling' here, but rather reliable and virtuous. 'stone' might also mean 'lodestone', which was used as a compass, adding to their reliability.

Unmoved, cold, and to temptation slow;

This line substantiates the previous line with the addition of a new idea. That these people are not, in fact, stone but real people able to be tempted, albeit slowly. There is, perhaps, a hint of Christ's temptation. Christ, of course, might be the ultimate "they" character.

They rightly do inherit heaven's graces,

An interesting line that has legal terms as well as religious. "These people" can "inherit" the graces from heaven by "right", but there is a suggestion that if they betray that right it could be withdrawn as in a broken contract.

And husband nature's riches from expense;

"Nature's riches" could be anything such as beauty, strength, intelligence or sexuality. The onus here is that "these people" must be good "husbandmen" or keepers of these qualities - or else they will be "expended" and ruined.

They are the lords and owners of their faces,

A curious and most beautiful line. "These people" keep themselves to themselves. They do not "wash their dirty linen in public". They have restraint and control of their emotions. "Faces" also must include their mouths and what they speak.

Others, but stewards of their excellence.

Now we have a new group of people - "others". They are all those people who are not the special ones lauded in the previous lines. They are seen as subservient creatures - servants, almost, or "stewards". They are people who look after and serve the special ones.

And that is the end of the section about "these people". It was actually just one sentence.

The summer's flower is to the summer sweet,

The poet now turns his attention to flower imagery to encourage the boy to improve his behaviour. The flower - or the boy - is a sweet joy to the summer. The summer - or the thing that has brought the boy to flower - could be the poet himself.

Though to itself, it only live and die,

The flower has no perception of the existence of summer. He does not attribute his qualities to anything but himself. He lives and dies in ignorance of the thing that made him beautiful.

But if that flower with base infection meet,
The basest weed outbraves his dignity:

Here comes the sting. If the flower goes down with a canker any old weed would look more beautiful and dignified. If the boy carries on his current behaviour any old homeless tramp would look good against him.

For sweetest things turn sourest by their deeds;
Lilies that fester, smell far worse than weeds.

The last two lines are from an old proverb. Sweet will turn sour by deeds. "Deeds" in this case is almost certainly heavy drinking, promiscuous sex and the company of dangerous people.