A music teacher teaching a boy the piano says he's discovered 'the Devil's Chord' ergo the boy gets excited. Enter Maestro (Jinkx Monsoon) who tells him that the boy is his son and as he's outlived his usefulness he kills him. Then thanks the teacher for being a genius as he discovered the chord and then sucks the music out of him, literally, figuratively, take your pick. Then takes a bite out of all music and removes it from the world.
Meanwhile, the Doctor (Ncuti Gatwa) asks ruby (Millie Johnson) where she wants to travel. She comes up with the '60's as her mother's friend loves The Beatles and she listened to their album. Change of outfits to the '60's era and they find themselves on the famous zebra crossing as they walk down it mimicking the famous album cover. At the studios they relieve the tea lady of her trolley and serve tea to everyone. ending up in the studio where the Beatles are recording. However they think the song is rubbish as it's about a dog and boring. In the tea room, they tackle Paul (George Caple) and John (Chris Mason) separately. John says he'll amount to nothing and Paul is disillusioned with the whole concept of music. Soon finding out that music has vanished everywhere. With a rooftop piano the Doctor gets Ruby to play a tune and Maestro enters and proceeds to stop them. Giggling just like the Toymaker from The Giggle. Also as you'll recall the Toymaker mentioned his legions and Maestro says the same later on in the ep.
They escape and hide in a basement. The Doctor is once again fearful and as he explained in Space Babies why he was scared then, it's a little difficult to imagine that he could flee again and feel this way too once more. He says that the last time he fought an "Elder God" his soul was ripped in half. He can't go though something like that again. Ruby comments she's still alive in 2024 and as he takes her to her future, everything is destroyed and was that snow falling again, or debris flying around. With no music, humans went to war and destroyed themselves. Again another harkback to Doctor Fourteen (David Tennant) in The Giggle and his saying how humans are capable of such destruction.
Maestro controls the environment and as they head back to 1963, they are trapped again and the Doctor tries to control her meddling with his Sonic, which works for a while, but Maestro's tuning fork turns out to be more powerful than his Sonic. Here it's revealed that she is the Toymaker's daughter. As people begin to play the piano, listen to music again, Maestro turns up and proceeds to end them and along with that any spark of humanity getting away from the dismal doom.
At the studios, the Doctor and Millie try to use music to lure Maestro there and play notes which will become a Beatles' song. However, Maestro fights them at every opportunity and soon the Doctor ends up trapped inside a drum and Ruby inside a double bass. As each note they played was matched by Maestro and her violin. Since there is one chord that brought her here and so matching that chord will take her back or vanquish Maestro. The piano is pushed out of the room. Which was handy as Paul and John pass by and play that very note needed and thus saving Ruby and the Doctor. Discovering they have the talent to make good music after all. Maestro is defeated and there's a song and dance number at the end, which quite honestly was a bit cheesy, "there's always a twist at the end." Twist as in story plot point, but also as in the dance, I presume, ha. There was also a dance number with the aliens in the Christmas Special, which seems to have carried on further here.
Some references to The Giggle here as well as the Doctor telling Ruby that he lived on Totters Street with his granddaughter, Susan, but she's gone now. Referencing the first episode, An Earthly Child. Ruby says she's still alive so the Doctor won't fail, only living her demise, so to speak in the future. As she should now, Time isn't set in stone and she's seen that by the Doctor taking the TARDIS anywhere in time. References to The Pyramids of Mars here.