O thou, my lovely boy, who in thy power
Dost hold Time's fickle glass, his sickle hour;
Who hast by waning grown, and therein show'st
Thy lovers withering, as thy sweet self grow'st.
If Nature, sovereign mistress over wrack,
As thou goest onwards still will pluck thee back,
She keeps thee to this purpose, that her skill
May time disgrace and wretched minutes kill.
Yet fear her, O thou minion of her pleasure!
This is a key line in the sonnet, a statement of the inevitability of time and the resulting decay. This is despite the fact that he remains “my lovely boy”. 
 The hard consonants continue. Note the  perfectly rhymed  “wrack” and “back”, and the internal  consonantly rhymed  “pluck” and “back”. 
 The essence of this line is that Time or Nature can play with her power; she may let him grow more beautiful (“sweet self gow'st” in line four), but ultimately she has the power to drag the boy towards decay and death.
Sonnet 126