Due to the plot of Crashing Down being a massive tangled mess, this article will abandon chronology and summarise by character and arc. It's all a lot better in the original, you know. Why not go and read it?
In June 2006 HST, two enemies from the PPC's past came back to haunt them: the Black Cats, formerly the DIS, and the Mysterious Somebody, leading his Mary Sue Factory. Fortunately for the embattled agents and Flowers, others returned to help fight them: Blue and Tango, Nyx and Dassie, the Nightshade and Orchid.
The attacks tore HQ apart, killing many, but victory was finally achieved, and the twin threats of the Cats and the MS were gone forever.
The Black Cats
The Black Cats capture Salamander and interrogate him, learning the current disposition of HQ. They then engineer a power cut by stacking canon books on the grave of J.R.R. Tolkien, temporarily stopping him spinning in his grave and cutting off power to HQ. With the power out, they disable the security systems that would prevent them attacking.
After finding evidence of Tango's theft of a remote activator, the Cats decide to push their invasion forward. Arriving in the Tomb of the Unknown PPC Agent, they designate their targets: the Board of Department Heads.
Twp'atwt attempts to kill the Sub Rosa, but is repulsed. Mkellin is similarly defeated by Hornbeam and DoSAT, but Flickerbright manages to kill the Wisteria. Lady Zhevago attacks and occupies DIA Central, giving the Cats a base of operations.
While the attacks continue, Twp'atwt is tasked to investigate a hammering on a door in Finance. He discovers the Factory assault team, and while running from them is killed by Blue. The remains of his team are wiped out by the MS.
Mkellin reports his failure to kill the SO, which sends Lady Zhevago off by herself to take him out. The remainder of the Black Cats head out to attack the Mysterious Somebody, but Mkellin and the Scouts defect at the last moment.
Nendil Morifëa and Flickerbright are killed in battle, by Tango and Kayleigh respectively, and the Bracket Fungus is taken out by Nyx and Dassie. Lady Zhevago is cornered in the SO's office by Selene, and then recornered by the MS. She defects to his side, but is turned by the SO and kills the MS, at the cost of her own life.
The Mary Sue Factory
Alerted by their monitoring program to the return of the Cats, the Mysterious Somebody builds up his army of Force-using Mary Sues and launches his own invasion. They suffer some delays, but eventually their forces set out – watched by the Nightshade and Orchid.
The pair of Flowers launch a raid in an effort to kill the MS, and enter the Factory with the help of the Ghost. They team up with the Gladiolus and capture the MS, but he escapes, killing the Orchid in the process. The MS joins his army for the assault on HQ, breaking down a door in the Department of Finance, and they win a major battle in the Large Auditorium against DAVD-led forces and the Black Cats.
The Nightshade and Gladiolus give themselves up to the Weeds, joining forces with them. The Security Departments launch an attack on the MS's army, but the Mysterious Somebody himself heads up to the SO's office. There he injures Selene and recruits Lady Zhevago, only to have her sacrifice herself to kill him. With his death, his armies collapse instantly. The Mary Sue Factory itself is destroyed in an explosion engineered by the Ghost.
The Security Departments
Following a tip-off by Kayleigh, the DIA aren't entirely unprepared for the arrival of the Black Cats. They alert the Sub Rosa, who fends off an attack on her with the help of the DES. The DES lock down HQ, keeping the Cats bottled up inside, but send in scouts to coordinate with the SO.
When DIA Central falls to the Cats, they retreat, and with the help of Intel and the Weeds, wind up outside HQ. The Sub Rosa pays a secret visit to Legal, but is unable to get much out of them (of course).
Members of the DES and Intel encounter the Nightshade and Gladiolus, and the pair rejoin the PPC. The combined departments then launch an all-out assault on the Mysterious Somebody's army. They briefly encounter Nyx and Dassie, but send them on their way and concentrate on the cleanup.
The Laison-Dimond Clan
During the power cut that heralds the beginning of the invasion, Steve finds himself trapped with Narto and Selene of DOGA. At the same time, Tango slips past the Black Cats and returns to HQ, bringing a warning. Unfortunately she encounters the DIA, and is promptly arrested, her warnings misinterpreted as threats. Trying to contact an agent she knows – any agent – she eventually hits on Steve's name.
Mortic and Elanor discover the invasion and attempt to warn Upstairs. They head out into HQ, after leaving a message for Steve, but are turned back by the Cats.
Steve attempts to visit Tango in prison, but when they learn of the invasion, they lock him up with her, since his sister Ontic is one of the enemy. He engineers their escape, but is cut off from his own RC. He receives Elanor's message and heads down to meet her. They discuss the events of the day, and decide to try and rescue Ontic.
The group encounter Dassie and Salamander, leading Tango to attack Dassie. She is interrupted by the appearance of Narto and Selene, which gives Dassie a chance to escape.
Some time later, the company encounter the battling armies of the Black Cats and Mary Sue Factory. They pull Ontic out from the thick of things, and Tango and Steve duel and kill Nendil.
Taking Ontic to Medical, Elanor and Mortic are ambushed by Blue, but warned by Nyx in time for Elanor to kill him. Ontic makes it to Medical, and is able to be healed – at least partly.
Vemi and Company
Vemi and Penny are caught in the power cut, and go to visit the Sunflower Official immediately after. They discuss the terms of their contract – as Discworld assassins, they are not technically employees of the PPC.
Morgan is warned by her TARDIS of the invasion, but is sent on a mission before she can find out what's wrong. On returning, she and Traf head down to investigate.
Vemi and Penny encounter the Cats, and then meet up with Traf and Morgan. The quartet head up to the SO's office, and are there when Nyx and Kayleigh arrive. They head off to attack the Cats, going through the DMS to pick up more fighters on the way.
With a small army of assassins at their back, the quartet corner Mkellin and the Scouts in the Cafeteria. In the ensuing battle, the Cats are (almost) all killed, as are Vemi and Morgan — but it doesn't stick for either of them.
Nyx Nightingale and Friends
Sparking off the events of Crashing Down, Salamander is captured by the Black Cats while on a mission. They interrogate him on the current state of the PPC, then torture and blind him.
After the Cats invade, Kayleigh encounters them in the halls. When they try to kill her, she mistakes them for DAVD, as in the DAVD Conspiracy Theory. She attempts to tell the DIA about Salamander's disappearance, but they get distracted by her mentions of the Cats. Kayleigh returns to Middle-earth to hunt for Salamander herself, and finds him in the company of Nyx and Dassie.
Nyx explains who they are, and learns that the Cats are back in HQ. After some debate, she and Dassie decide to return and offer to help. Back in HQ, Dassie takes Salamander to Medical while Nyx and Kayleigh head up to the SO's office.
Leaving Medical, Dassie and Sal run into Steve, Mort, Elanor and Tango, and Tango identifies Dassie as a former (or, to her, current) Black Cat. She attacks him, but the arrival of Narto and Selene by portal lets Dassie escape, fleeing towards the SO's office. Salamander is left with Narto, as Selene heads off in pursuit of Lady Zhevago.
Nyx and Kayleigh arrive in the office, and Nyx offers her assistance. She correlates the reports of Black Cat activity, coordinating the PPC's efforts, until Dassie arrives. Then Kayleigh runs off to find Salamander, while Nyx and Dassie head down to the battle.
Kayleigh kills Flickerbright in her own inimitable fashion, while Nyx and Dassie encounter the Security Department forces on their way to the fight. They are allowed through, and contrive to kill the Bracket Fungus. After the battle, they are able to warn Elanor Laison of Blue's attack on Ontic, and eventually rejoin the PPC.
Shortly before the Cats invade, Blue returns to HQ by plothole, landing in the Tomb of the Unknown PPC Agent, where he encounters the ghost of Imbolc Telyan. She warns him of the imminent arrival of the Cats, forcing him to flee into HQ. In hiding, he chooses (possibly prompted by the Ghost) to hunt and kill the Cats who hurt him most – Twp'atwt and Ontic.
Blue encounters Narto and Selene, who, unaware of who he is, join up with him. The trio attempt to attack the Cats in DIA Central, but are repulsed, and Blue gets separated. He locates Twp'atwt near Finance and kills him. After the battle is over, he finds Ontic in the hands of Elanor and Mortic. Trying to kill her, he is himself killed by Elanor – but in death, he is reunited with Imbolc.