What's he saying?
"Cupid laid by his brand, and fell asleep: / A maid of Dian's this advantage found,"
Cupid set his torch aside and fell asleep. A maid of Diana's found this to her advantage,
"And his love-kindling fire did quickly steep / In a cold valley-fountain of that ground;"
And quickly plunged his love-kindling fire into a nearby cold fountain from the valley,
"Which borrow'd from this holy fire of Love / A dateless lively heat, still to endure,"
Which acquired from that holy fire of Love an eternal, active heat that still endures,
"And grew a seething bath, which yet men prove / Against strange maladies a sovereign cure."
And which grew into a bubbling bath, which men still try out as an almighty cure against unusual illnesses.
"But at my mistress' eye Love's brand new-fired / The boy for trial needs would touch my breast;"
But Love's torch regained its fire from my mistress's eyes, and Cupid, to test it out, touched my chest with it.
"I, sick withal, the help of bath desired / And thither hied, a sad distemper'd guest,"
I, ill as a result, desired the help of that bath, and hurried there as an unfortunate, upset guest,
"But found no cure: the bath for my help lies / Where Cupid got new fire-my mistress' eyes."
But it did not cure me; the real cure for my disease lies where Cupid got his new fire: my mistress's eyes.
What's he saying?
This sonnet and the one that follows tell a similar tale - that of Cupid, the bringer of love, whose instrument of passion once was stolen by one of Diana's nymphs. Diana, the goddess of the hunt, was said to be a chaste goddess whose female attendants (nymphs, or votaries in sonnet 154) were likewise expected to be devoutly abstinent.
In keeping with this calling, in sonnet 153 a nymph catches Cupid fast asleep and decides to extinguish his "holy fire of Love." She attempts to do so by drowning it in a nearby cool fountain, but the fire burns so strongly that the water of the fountain absorbs its heat and becomes a bubbling bath, which is from then on believed to have special healing powers.
In the third quatrain, Cupid relights his torch with the eyes of the dark lady and tests it out on the narrator, who as a result falls victim to the burning disease of love; cf. "My love is as a fever" (sonnet 147). (Note that in sonnet 154 the narrator similarly does not appear until line 12.) The narrator tries to cure himself by visiting the bubbling bath but fails, and instead discovers that the only cure for his disease is to be found directly at the source - his mistress's eyes - for it is only there that he can quench his fiery passion.
It is often said that sonnets 153 and 154 do not fit well with the overall sequence. Whereas the rest of the sonnets deal primarily with the emotions, endeavors, and experiences of the narrator (real or not), these sonnets are instead built around mythical events that are tied in with the situation of the narrator only as the sonnets come to a close. Both of them do, however, make mention of the narrator's mistress - if only peripherally - and for this reason they are generally included as part of the dark lady sequence.
Furthermore, some scholars have suggested that these two sonnets are in fact duplicates, two versions of a single sonnet that the poet had intended to choose between but perhaps never got the chance to due to their unauthorized publishing. The similarities in content and form between the two sonnets are indeed suspicious: we can read 154 almost as a direct paraphrasing of 153 (or vice versa). It is interesting to ponder over which of these sonnets may have been written first and why the poet tried again.