Summary: Act 2

After the Jellicle Ball, the cats are resting. Jellylorum helps Gus the Theater Cat, a frail, elderly tom, onstage, and he is welcomed and given a place to sit. Old Deuteronomy, who is still thinking about Grizabella, sings of "the moments of happiness" in sophisticated terms that no cat quite understands. Coricopat and Tantomile receive the message and pass it on to Jemima, who sings it more simply. As one, all the cats rise to their feet and stand in unity.
Gus goes to pay his respects to Old Deuteronomy, then returns to sit down. Jellylorum sings of his past as an actor, and Gus claims that no "modern production" is complete without him. "His grandest creation...was Fireforefiddle, the Fiend of the Fell", and he has a flashback of the dark cat. In some productions this flashback is a song in itself, "Growltiger's Last Stand", however since the movie came out it is more often cut short, replaced by Gus overwhelmed by emotion and taken offstage by Jellylorum.
Old Deuteronomy wishes to introduce Skimbleshanks, the railway cat, but the said Skimbleshanks is nowhere to be seen...he is fast asleep. Once found and woken, he talks through a train journey and describes all his various duties. Towards the end various materials are found to build a mock train.
As Skimbleshanks climbs up on the wheel to take his bow, a third crash sends the creation toppling to the ground. The evil Macavity leaps onto the highest point of the stage, laughing demonically, and hypnotizes the cats while his dark minions abduct Old Deuteronomy. The toms run after them, while the she-cats go into hiding...all but two, Demeter and Bombalurina, who share firsthand their experiences with Macavity and his frightening capabilities.
The toms return immediately before Macavity, who is disguised as Old Deuteronomy. Only Demeter and Bombalurina recognize him. Bombalurina tries to protect Demeter, but is thrown off. Demeter pushes the gathered cats away from Macavity and pulls his disguise off. Macavity chases the different cats and tries to abduct Demeter, but  is stopped by Alonzo and fought by Munkustrap. Eventually all the Jellicles begin to gang up on Macavity, who creates an electrical explosion that cuts out the stage lights, and then escapes. The Jellicles shine a spotlight on where he was and repeat his signature line, "Macavity's not there!"
In the film version, Munkustrap and Demeter kiss after Macavity's disappearance, causing most fans to believe they are mates...however, this does not happen in most other productions. The cats return to the task at hand: "We have to find Old Deuteronomy!"
The Rum Tum Tugger suggests that they ask the magical Mr. Mistoffelees for help, and shares how clever the young cat is. Mr. Mistoffelees arrives, restores the lights, performs various magic tricks and dance moves, and with the help of Cassandra or (very occasionally) Demeter, brings back Old Deuteronomy.
Jemima announces that daylight is coming, and Munkustrap says that the time has come for Old Deuteronomy to choose which cat will go to the Heaviside Layer. Grizabella returns, and most cats go to the side of the stage, ignoring her or challenging her, still afraid. Victoria for a second time tries to approach her, but is again prevented by Jellylorum. With encouragement from Old Deuteronomy, Grizabella begins to sing and is ignored at first, but eventually the cats listen to her. Rarely, a flashback version of Grizabella dances while she sings. At one point she stumbles and falls, but Jemima encourages her not to give up. At the end of Grizabella's song, Victoria goes to her and touches her hand.
One by one, each cat accepts Grizabella and welcomes her back. It is unanimously decided that she should go to the Heaviside Layer. Old Deuteronomy takes her part of the way, and then she finishes her own journey with farewell and encouragement from all the Jellicles.
In concluding, Old Deuteronomy explains to the human audience that cats need to be "ad-dressed" with much respect.