Only one tenth of the mass of the universe is accounted for.
The other nine tenths is the paperwork

Three times a man knocks on Nanny Ogg`s door requesting the help of the world`s greatest midwife; once when was a young woman, once when she is mature, and finally in her old age when she responds to the call.

Jeremy Clockson was a foundling delivered onto the steps of the Guild of Clockmakers. He is now a clockmaker in his own right. A very good clockmaker who can wake up at 6:59 in order to check his alarms go off at 7:00. He has a customer, Lady Myria LeJean, who is willing to pay well for the world`s most accurate clock, the one that measures a quantum of time. She leaves him with a copy of Grim Fairy Stories, pointing him towards the Glass Clock of Bad Schustein, He also take delivery of a helper. An Igor from Uberwald has posted himself to Jeremy, again all paid for by Lady LeJean. Igor is skilled in the glass working and electricity required to build a glass clock like the one in the story. Igor tells Jeremy that his grandfather helped build the Bad Schustein glass clock; it is a real story. The lady makes daily visits to see how work is progressing, but Igor is having doubts about her. Important parts go missing when she visits, delaying the work, and she can vanish into thin air when Igor follows her.
The 60 Bizot stamp from Uberwald with the
double-headed bat emblem

Another of the Igor Thtitch-Up thtampth

At Frouts Academy Susan Sto Helit is proving to be a strict but inspiring teacher, much against the beliefs of Miss Frout. In order to teach her young class how to tell the time, with a click of the fingers they can be hovering in front of Clock Tower of the Unseen University at 12:00 and then, click, in the central square in Genua at 3:00. Susan is summoned by Death and told about the glass clocks. Creating such a clock ends time itself. The first clock did manage that but a spring broke and history had to be repaired. Death himself cannot be directly involved. Susan does some checking at the Guild of Historians and sees that many discrepancies are glossed over. Just how many Battles of Koom Valley were there, and how did Omnia lose an entire century? Susan, with the help of Death of Rats and Quoth the Raven, needs to find whoever is making the clock and that means meeting the midwife. Nanny Ogg is reluctant to divulge secrets at first but reveals the mother was Time herself, and the father was Wen the Eternally Surprised. Furthermore it was twins - of a sort.
Susan Sto Helit five pence

        
Guild of Historians three pence  The Koom Valley pair show Dwarfs chasing Trolls and Trolls chasing Dwarfs. A postcard from Genua
Issued in the Year of the Signifying Frog
  
The Death of Rats and Quoth the Raven Post Early for
Hogswatch postal label.

The Monks of Wen the Eternally Surprised, better known as the History Monks live in the mountains near the hub, at Oi Dong. Lu-Tze, the sweeper, has a new apprentice, Lobsang Ludd. He will be shown The Way of the Sayings of Mrs Cosmopolite. Lobsang was also a foundling, his guild being the Thieves Guild. He was recruited to the monks by Soto, and shows a certain promise. Lu-Tze takes him to the Mandala where Lobsang sees a disturbance in the coloured sands below the normal swirls. Suddenly there is an alarm. The spinning procrastinators, which share out time across Discworld, are going wild and time is breaking up.

A letter to Mrs Cosmpolite from Zlobenia
asking for words of wisdom

The Thieves Guild building on the original
design for the 4p stamp

Lobsang instinctively knows how to control the procastrinators which can wind time in and out, moving from where its loss would go unnoticed to where it is needed,. The abbot knows that a new glass clock is being constructed. Lu-Tze went to Uberwald last time and was just too late to stop it being started. After the spring broke huge chunks of history had to be moved around to patch up the cracks. The Abbot forbids Lu-Tze to visit Uberwald this time. He sets off to Ankh-Morpork with Lobsang instead, as this is where he feels the clock is. They do not have much time and so they slice time and borrow a broomstick to get there as fast as possible.
Behind all this are the Auditors, who measure everything in the universe. The last thing they want is humans etc doing unpredictable things. If time was to stand still they could get on with their task. Lady LeJean is one of them, but she has started to take on human traits and emotions having spent so much time in human form. On the day that the clock should be ready six more auditors take human form, taking colours as names, to visit Jeremy. Jeremy is already to go, but Myria urges caution, while Igor bemoans lack of decent thunderstorms. However one storm approaches. Myria tries to reach for a hammer to smash the clock, but is prevented. Igor`s battery sends a bolt into the sky and a big lightning bolt starts to form. Lu-Tze and Lobsang see this happening and slice time at an incredible rate, too much for the sweeper, to get to the shop. Lobsang lurches through the window a fraction too late. The clock starts - and time stops.
Death has been trying reactivate the Four Horseman ready for the upcoming apocalypse. Pestilence is found in a hospice in Llamedos spreading a touch of flu, while Famine is sampling the local cuisine in Genua. Neither is keen to ride again. War is told by Mrs War that he does not want to ride again. They eventually agree to take on the non-human auditors - they will ride again at the apocalypse, but it does not say against who Chaos` flaming sword comes into play.
Lu-Tze finds himself in dairy. Thanks to the portable mini procrastinators they picked up from Qu before leaving time does not stop for him. Neither has it for Ronnie Soak, the milk delivery man who always supplies fresh dairy produce always on time. The sweeper works out that Soak is Kaos backwards and Ronnie is the fifth horseman, Chaos. Lu-Tze uses the same talents he uses on the abbot to get Ronnie`s spirit fighting back.

The first Llamedos (mae hi wastad
yn pistyllia) Ffyrling stamp

Genuan cuisine features on
this $1 stamp

Susan finds Lobsang outside the shop and together they have to find Jeremy. Susan using the skill of one from DEATHs Domain locates him in the Art Gallery. He has been taken there by Myria who has laid down a number of auditor traps. There are auditors in human form all over Ankh-Morpork and they are beginning to bicker. There are over 700 auditors, but there are only 104 different colour names known to them. There are arguments as to which colours take precedence and whether dark avacado is  real colour name. Myria knows what can defeat those in human form - chocolates. The information input when they eat one is just overwhelming. They gather ammunition at Wienrich and Boettcher, Susan sampling some just in case and she renames Myria as Unity - from many to one. They need to get Lobsang to the clock shop and Susan can smash the clock. Susan knows the truth; Lobsang and Jeremy are the same person, born twice when time split. Their mother was Time herself, and their father was Wen the Eternally surprised. She smashes the clock, and Lobsang and Jeremy merge into a complete person.

The Fourpenny Horizontal issued by the Guild of Seamstresses features
a painting that may well have hung in the Royal Art Gallery

Lobsang finally meets his parents, and takes over from his mother as Time. In Oi Dong all the procastrinators are broken of still, apart from one. Lobsang gets them running again, one by one, repairing history as they come back on line. Lu-Tze still still insists that Lobsang has to be formally released from his apprenticeship.