I’ve been saying for ages that I was going to sit down and write all the reasons I love Scherzo. It’s on my top ten list of Doctor Who stories. I adore this story. I would like to preface this with several things:
1. I’m a bit batshit about Charley. I know and acknowledge this so hopefully I don’t let the crazy out too bad.
2. Well I like Charley/Eight I’m not batshit about it. If given the choice between two stories, one Eight/Charley and one something else I really like (Peri/Erimem, Six/Evelyn, Ace/Hex, etc) based only on the pairing and told I could only read one I would probably choose the other one.
3. If given the choice between a Charley story and someone else I would choose the Charley story.
4. I think Scherzo is the third part of a trilogy started with Neverland and then going to Zagreus.
I think these are important things to consider before discussing Scherzo. 
I think the first thing to mention when talking about Scherzo is the fact that Charley stowed away on the TARDIS to go with Eight. He didn’t want her to go with him into the Divergent Universe, something he had to do. He wanted Charley to stay safe in our universe and had asked Romana to look after her. Now the question is why did she stow away?
I think there are several factors involved. The most obvious being that Neverland and Zegreus had both Charley and Eight telling each other that they loved the other. And in the romantic in-love sense not the friendship sense. So we have that. Scherzo is in one sense the cumulation of the Eight/Charley relationship. They do just about everything you can ask for in a relationship in this audio and after Scherzo they go into a weird just friends place. I think this is because the Big Finish writers wrote themselves into a corner here and couldn’t find a way out so they just pretended that it hadn’t happened. Which makes for some weird dynamic later, but for the purpose of Scherzo there is a brilliance in the Eight/Charley relationship.
The next thing to consider about Charley choosing to go with Eight is that she has no one else in the universe. Literally. By the end of Neverland Charley understands, as much as anyone who’s not a Time Lord can, the importance of the Web of Time as well as how fragile it is. This means that she knows she can never go back to her time. She can never see her family again. Not without putting the Web of Time in danger and Charley knows that she can’t do that. She had a very eloquent and passionate speech about just that in Neverland.
Charley is an adventurer. Unlike a lot of people that the Doctor picks up Charley was already on the search for adventure when she runs across the Doctor. She’s on a journey as Charlotte Pollard, Edwardian Adventuress. She loves the danger and the excitement and the discovery of new things. If she could have lived without meeting the Doctor and had done so she would have been traveling Earth in search of adventure. It’s what she was wanted to do.
At the end of Zagreus Charley is sitting with Leela discussing going with the Doctor to the Divergent Universe and she tells Leela, “The choosing is easy. It’s the leaving that’s hard.” I think this means that choosing to leave was easy, she loves him and he needs someone at that point in time. The leaving is hard because it’s leaving behind everything she knows. But she’s already done that, she can never go home again. And going to another universe is about the biggest adventure you can have. So Charley leaves.
Now the beginning of Scherzo. They have entered the Div!Universe and the Doctor is still unaware that Charley is there. He has gone a bit insane at this point. He never quite regains his sanity throughout the Div!arch, but it’s at it’s most pronounced here. He’s ready to give up and die and it’s Charley who saves him. She’s the one that drags him out of the TARDIS before they can die and they enter into their first world of a new universe. And then Eight does one of the best things ever.
He yells at Charley. He screams at her. For a period of time he refuses to believe that she’s really Charley (see: insanity). Because Charley wouldn’t have betrayed him like that. Everything he did in Neverland and Zagreus was to keep Charley safe and she wouldn’t have betrayed him like that. He is pissed at her. And they have a huge fight over it. Charley argues that she had the right to choose to come with him, she loves him and he loves her and he needed her. And she’s young and still a bit naive so those are good enough reasons to her. Eight is still vastly annoyed with her and he stays annoyed with her until the end of Scherzo. I love that there was this consequence for her actions. She wasn’t patted on the head and greated with a smile. She was yelled at and forced to justify her actions. I love it.
Then there is the setting. A featureless (as far as we can tell) landscape with a light so bright it blinds them. So throughout this they are forced to hold hands with each other to keep from getting lost. It’s almost complete sensory deprivation. All they have is sound. Accept for those occasions where the light dims and feeling returns and they get to eat. So they walk along this landscape (later revealed to be a circular tube in a laboratory) holding hands and eventually sussing out their relationship. Such as it is.
There is also a noise creature in this strange new world. It’s important later.
Eventually Eight accepts Charley’s reasons for coming with him, he’s not happy about them but he accepts them. They spend a lot of time in this place. They don’t know how long as they lose all track of time. At one point in time Charley becomes complacent in this new life they have. She talks about how it’s not so bad. It could be worse and really all things considered it’s not bad. Eight then has a lovely little piece of dialog about how she can never forget that they are in a prison. Her mind is tricking her into thinking the way she is and she should never not be angry that they are stuck there. Later Eight has a moment of false hope, thinking he’s found a way out, and it’s Charley that pulls him back and makes him face reality. It’s brilliant. They save each other. Neither one could have survived without the other one there. And I love that. I really do.
Now back to Mister Sound Creature. He is in a weird way their child. He came into existence because of them and they fed him and kinda cared for him in a weird way. Only now he needs to kill them in order to live. So in order to fight Mister Sound Creature Eight and Charley merge into one person. Literally. It’s kinda creepy, but it works. Now MSC has to convince Charley and the Doctor to agree to die for him. Charley says no. Eight says yes. There is convulted troll logic used here.
So what does Charley do? She bursts in and tells Eight that because he can’t say no she’s going to do it for him. So she saves him once again. So the finale battle to fight their weird child thing is won because of Charley. I like any story where the Companion saves the Doctor and the day. One where the Companion does so while merged together with him to destroy their weird child thing? Wins.
And then we have the Doctor asking Charley to stay merged with him. To stay locked together forever. And Charley again says no. She’s the one that tells him they can’t do that. She’s the one that backs away. Again win.
So by the end of Scherzo we have what could be seen as an entire relationship between Charley and Eight. They make up, again admit to loving each other, have a child and then break up. I love that aspect of this story. As I said the writers had backed themselves into a corner with the Eight/Charley story. They didn’t want to(couldn’t?) go forward and they’d gone too far to go back. So the writers essentially did what could be seen as a relationship in all it’s forms in one story. So as far as the Eight/Charley arc goes it was the best conclusion we were ever going to see for it.
Then we have Charley saving Eight. I love that. I love that she saves him as much as he saves her.
Eight being bloody well pissed at Charley for sneaking aboard the TARDIS. Yay for consequences!
Charley being Charley and following her heart and her sense of adventure.
Really there is nothing about Scherzo that I don’t like. Hopefully I conveyed that well in this little essay.