It's kind of wild that BF built an empire on the back of this story. It's mostly just a showcase for the three Doctors, but curiously Seven has the least interesting story and personality out of all three little mini adventures. Usually I think of Five as the most personality-less but here we are. Five actually gets a fun little time trying to pretend to be a German spy and then distract the siren. Six gets to solve the plot at the end, of course, saying that he's the most practical of his incarnations when he condemns a giant time whale to eternal torment to fight the sirens (something the time whale asks him to do). He even gets to be a little rude and confrontational, though he's already massively toned down from his TV episodes.

This also functions well as an actual argument for why changing time is bad. Instead of just "laws of time" or "fixed point" there's actual consequences that are felt by the Doctor's accidental changes. That's how "changing time bad" stories SHOULD function but often it comes off as lazy to not give a real reason. I also find it interesting that the pseudo companion the story gives them all is the same person every time but also the villain.

Even though it's about changing timelines, it's a relatively simple little story about the Doctor accidentally changing history so that the Time Lords get destroyed. Not bad, just not great. Still, a decent little start. Makes me look forward to the great stories I know that are to come.

I find it interesting that Paul Cornell brought this sirens that eat changes in time back for Father's Day. In that episode there's no real reason why saving Rose's father is bad other than the sirens will eat them. But here there's actual reasons given and the sirens are incidental, manipulating bad changes in time, which I like.

Also, I just find it funny that they use the "contact contact" sound effect from The Three Doctors. I didn't expect it and it made me laugh.