This episode starts with the group rushing Phoebe to the hospital in order to give birth to triplets; the father is her brother (in joke, Phoebe's acting as a surrogate.) Monica and Chandler are late (twice) because she forgot her jacket (that is, the two of them were having a quickie). And not long into the episode, Joey is going to start having 'sympathy pain' for Phoebe (he'll be diagnosed with kidney stones: "Up? Up?! Nothing's going up... what's a urethra?")
But the fun really begins when Phoebe's doctor slips in the shower ("What an idiot!") and her replacement physician turns out to be a HUGE Fonzi fan ("from Happy Days. Fonzi. The Fonz." "Okay.") Since the best Ross can do is find a "Doogie" Howser lookalike, Phoebe is stuck with this doctor and worries only when Rachel makes an offhand comment about liking Mork (setting off the doctor since Mork froze the Fonz on a Happy Days show.)
As in many episodes of Friends, though, the main action of the show will resolve around three groups of the friends: Ross stays with Joey to help him while the kidney stones pass (a major parallel to how far along Phoebe gets in labor, all the way to the point where the stones have passed, and Ross and Joey note how small the stones turned out.) Phoebe asks Rachel to find out from her brother if she can keep one of the kids ("oh my God - I'm going to be in the news!!") and Monica and Chandler are dealing with his statement that the two of them are just "goofing around."
Happily, as it turns out, Chandler's dictionary defines "goofing around" as "two friends, who are great together, and have amazing sex, and just want to spend more time together" and the two work out their differences. As mentioned, Joey manages to pass his stones (without breaking Ross's hand), and Phoebe gives birth to three healthy children (all of whom her brother is going to keep).
The show ends with a heart-breaking scene of Phoebe talking to the triplets about how she has to give them up -- it hurts her worse than labor -- and she gives all three babies tiny high fives.
This is one of the few episodes where Phoebe is truly three-dimensional (and not just a two-dimensional characterization of a slightly dippy blonde.) She shows a sensitive side of her personality which is rarely seen (except occasionally with Ross.)