Look, I know that just because the Doctor does a guest appearance on Sarah Jane’s show that does not mean he is going to save the day. It’s still her show, so either she saves the day or one of the kids do.
That doesn’t change the fact it is a real treat to see him here.
So, to recap, Sarah Jane met some dude named Peter and is all set to marry him, but the Doctor came by to object because the Trickster wanted it to happen. The Trickster can play with time, so he traps Sarah Jane and Peter in one reoccurring second and the Doctor, K9, Luke, Rani, and Clyde in another. Now, the Doctor could save the day if he had the TARDIS…but the TARDIS is blocked from entering the plane.
None of that explains why the Trickster wants Sarah Jane to be happily married. Peter is, legitimately, her soulmate. It does make sense, though, if a happily married Sarah Jane stopped doing stuff to protect the Earth. Then…CHAOS!
That is what the Trickster wants.
The Doctor does eventually get the TARDIS to come back long enough for him to get on, but a strong wind keeps the kids out. Clyde is pulled close enough to touch it, giving him some kind of energy shock, and making him realize he can hurt the Trickster.
That actually works! For a second.
It does knock Clyde to the same second Sarah Jane and Peter are in. And an injured Clyde gets someone to act…Peter.
See, Peter thought the Trickster was an angel and…
…wait, he thought the being with no eyes and a mouth full of pointy teeth was an angel? Man, he may have been more messed up than we thought…
Anyway, he was dying, and he was given a chance to live and meet the woman of his dreams. But seeing how important she and her friends are to saving the world, Peter calls off the deal and the Trickster is defeated.
Then everyone (except the Doctor) goes back to the wedding, but the groom is missing since he’s, well, dead.
The Doctor does pop up again, complete with some hints his time is coming to an end (because this Doctor’s time was coming to an end), and a lot of references to old episodes involving Sarah Jane and other Doctors she knew. The kids get a tour of the TARDIS, and it was a nice guest appearance even if it was a sad story for Sarah Jane.
Because Peter was the right man for her, and she barely got a chance to get to know him.
But some things are bigger than one woman’s happiness, and she still has her crew. That’s something that puts her a bit ahead of a lot people…including the Doctor.