We learn right away what Della's "quest" is: to find the perfect present for Jim. We also know the problem she faces: she doesn't have the money to buy the perfect present. Della is frustrated by this and cries. Afterwards, she looks at her hair, and rushes out of the flat. We get the sense she has made a fateful decision.
Della remedies her initial problem with an epic sacrifice, and continues on her quest for the perfect present. It takes two hours to find it.
Della finds Jim the watch chain and gets it. Mission accomplished. Only now there's a new frustration: she realizes that Jim may be more unhappy about her missing hair than pleased by his present. She tries to make the best of her shortened hair with the help of a curling iron, though she now looks like a schoolboy. She prepares for Jim's return.
Jim arrives and puts Della through a scare – he's shocked by her hair, and she doesn't know what to make of his reaction. He recovers and tells her to open his gift. She does, and finds the combs. Della is both moved with joy and racked by sadness since her hair is gone. It remains now to give Jim his present. Will he like it?
Jim at last receives his gift from Della, only to tell her that he has sold the watch for which it was intended. The action of the story stops there, and we might feel a bit ambivalent (that is, we have two different reactions): what's happened is touching, but was Della's now-useless present actually a success? Was her quest successful? Luckily, the narrator is there to take away those nasty second thoughts and assure us that yes, it was, since her gift (along with Jim's) was the wisest of all.