You remember Berry Rydell, right? Ex-cop, now security operative for hire, last seen in Virtual Light heading off into the sunset with sexy ass-kicking bike messenger Chevette Washington, having foiled some slightly incomprehensible plot involving using nanotechnology to regenerate areas of earthquake-devastated San Francisco.
You remember Colin Laney, right? Gifted with the ability to "see" patterns in the flow of online data, last seen in Idoru helping to facilitate some slightly incomprehensible business involving the titular idoru, a female AI construct called Rei Toei.
Some unspecified amount of time has passed. Chevette Washington and Berry Rydell have had a bust-up and gone their separate ways, Chevette to live with a rich and punchily abusive new boyfriend and Rydell back to his old job as door security. Meanwhile Colin Laney is spending almost all his time immersed in the virtual world, living in a cardboard box at a Tokyo railway station, looking for patterns and connections in the data as his physical body atrophies rankly around him.
Laney has become obsessed with a shadowy PR guru called Cody Harwood, who he is convinced is shortly to have some key involvement in what Laney calls a "nodal point", a key moment in history, recognisable as such to most mere mortals only in hindsight. For reasons that no doubt seem completely obvious to Laney (but not so much to everyone else, the reader included) he decides that what needs to happen is for Berry Rydell (who Laney met, briefly, in the early stages of Idoru) to go to The Bridge (you'll recall from Virtual Light that this is the remains of the San Francisco-Oakland Bay Bridge rendered impassable to vehicles by the last earthquake) and act in such a way as to come to the attention of a certain individual. It turns out the "coming to the attention of" bit is fairly simple, it's the staying alive afterwards bit that might be difficult, since the individual in question turns out to be some sort of Taoist assassin, a bit like Yoda but taller and less green (and without the verbal tics) and wielding a Japanese tantō instead of a light sabre.
Meanwhile Chevette has escaped from her abusive boyfriend, and with the help of her Australian friend Tessa has temporarily holed up at Chevette's former place of residence, The Bridge. Inevitably she hooks up with Rydell again, but there's no time for any gooey romantic stuff, as it appears there are a large number of people out to kill him, and who if they can't take him out individually are quite prepared to torch the entire bridge and everyone on it. Fortunately the dagger-wielding assassin is on Rydell and Chevette's side, and turns out to be pretty useful with assorted other weaponry as well. Rydell has another ally in Rei Toei, who makes a crucial intervention during the climactic showdown to foil whatever it was Cody Harwood was trying to bring about.
What exactly Cody Harwood was trying to achieve is never very clear, beyond that it was something to do with nanotechnology, but, just as with the magic sunglasses in Virtual Light, that's largely irrelevant anyway. All Tomorrow's Parties is nominally the third book in the Bridge trilogy, Virtual Light and Idoru being the other two, but actually since those two were barely related to each other it reads better as simultaneously a sequel to each of them, the whole trilogy then forming a sort of two-pronged fork, a Y-shape if you will, rather than a straight line.
That said it's much more similar to Virtual Light than Idoru, and you could criticise it (as this New York Times review does) for being basically a bit of a rehash of the earlier book with a few elements out of Idoru thrown in. The plot doesn't really make sense, and it doesn't really give Chevette in particular enough to do other than be rescued from peril a few times. The ineffably cool knife-wielding assassin (who it turns out is called Konrad) is really the star of the show here, along with the brilliance of the basic concept of the Bridge as high-rise semi-anarchic shanty town.
As always Gibson throws in some prescient stuff involving technology - Tessa's relentless filming and documenting of her own life and adventures using a miniature camera attached to a remote-controlled airborne drone seems bang up-to-date, even though the book was published in 1999.
So, as always, if you must have one and only one Gibson it still has to be Neuromancer. If you want one other it'd have to be Virtual Light, but I'd recommend you read all of them.