Summary: Act 1

The moon rises over an out-of-the-way alley (presumably in the middle of London, although some people would probably argue with me, and even I changed it to fit the requirements of a Cats fanfiction I wrote). One by one, Jellicle cats enter the stage, and sing about their achievements. We meet most of the cast this way. Unusually aware of their human audience, the Jellicles take time to explain things, such as how cats are named, although they are scornful by how little humans know.
Once they have done this, they slip away into the shadows while Victoria, a white kitten, performs a solo dance. She is joined by Quaxo, who calls the cats back together. Munkustrap explains to the audience that "Jellicle cats meet once a year, at the Jellicle Ball" and that the Jellicle leader, Old Deuteronomy, "will soon appear to make what is known as the Jellicle choice". Just before dawn, he will choose one Jellicle to ascend to the Heaviside Layer and "come back to a different Jellicle life". What exactly this means is also up for interpretation, although many people assume it means dying, being reborn into a life without suffering, and it is clear that it is an honor to be chosen for this.
As the introduction ends, the cats begin to mingle and exchange news. Munkustrap introduces Jennyanydots, a middle-aged cat who "sits and sits and sits and sits" during the day, and at night keeps charge of the mice and the cockroaches that live in her human family's house. Demeter, Bombalurina and Jellylorum also help to introduce Jennyanydots, and the other cats present, especially the young ones, act out the roles of mice and cockroaches.
As her song ends Jennyanydots is praised and welcomed by the other cats, but is cut off by the arrival of the wild and seductive Rum Tum Tugger, who announces proudly that his "disobliging ways are a matter of habit" and that "he will do as he do do and there's no doing anything about it". He gets the young cats very excited, especially the kitten Etcetera and the red queen Bombalurina. The older cats are more disturbed by his antics, Demeter physically clawing him when he comes too close. His number ends abruptly as Grizabella, an old, outcast cat, enters.
Although the younger cats are interested in Grizabella and want to welcome her, they are constantly blocked by the older cats. Demeter shares that Grizabella "haunted many a low resort" and was once beautiful, known as "the glamour cat", but is now old and close to death. Hurt by the cats' treatment of her, Grizabella leaves.
Jennyanydots introduces Bustopher Jones, the "Brummel of cats". He is well-known at London's most elitist clubs, and is very popular among the older cats, although the kittens seem turned off by the fact that he is "the twenty-five pounder" and "putting on weight every day". As Bustopher presents Jennyanydots with a rose, a loud crashing and flashing red lights interrupt the gathering. Demeter identifies the cause as Macavity. Although it is not known who he is until later, his presence seems to upset her very much. All the cats run for shelter.
A high-pitched giggle signals the entrance of Mungojerrie and Rumpleteazer, a "notorious couple of cats". They "have an extensive reputation", with knowledge of many different trades, although their favorite is burgling or breaking things. As they celebrate their cleverness, the cats run on from different sides and throw their illegal bundles offstage, taunting, "And there's nothing at all to be done about that!"
While the cats rest, Munkustrap and the Rum Tum Tugger share the story of Old Deuteronomy, the wise Jellicle leader, who arrives at the end of the song and is much welcomed by everyone. Immediately following this, the cats act out a story of dogs fighting in the street and being scared away by the Great Rumpus Cat. Old Deuteronomy begins to speak about Jellicle Cats and Pollicle Dogs, but is cut off by another crashing. This time a glimpse is caught of Macavity in the shadows of the stage.
Again all the cats run for shelter, but once Munkustrap has checked that all is safe are slowly brought back to join in the "Song of the Jellicles", where again the Jellicles speak of what they can do and what they are like. This is followed by the official Jellicle Ball, an extended, many-parted dance number that includes territory-marking, a large mating dance led by Bombalurina and the Rum Tum Tugger, whirling and acrobatics, and a solo mating dance performed by Victoria and one of the toms--who it is varies, but usually Plato, Admetus, Alonzo or occasionally Quaxo.
At one point during the Jellicle Ball, Grizabella attempts to return, but is chased away. She returns again at the end of the Jellicle Ball, and is hissed at or pushed around by the other cats, who eventually leave her onstage. Jemima tries to speak on Grizabella's behalf, but is ignored. Alone with the exception of Old Deuteronomy, she tries to dance, but is too old to do so without causing herself pain. Feeling lonely and hurt, she gives vent to these emotions by singing, and adds that she remembers the old days, when she was beautiful and loved and happy, but that she cannot change what has already happened. She exits again, leaving Old Deuteronomy in tears and confusion, longing to help her but not sure how.