Because there are only three parts to Delta and the Bannermannys we are now already at the middle of the story! While there are some two-part stories in Doctor Who that are even shorter, three parts is shorter than the great majority, and it is not helped here by having a significant recap of the end of part one - it takes two minutes before we get back to the cliffhanger, as both Mel and the Doctor's separate situations are reprised.
Gavrok blows up the suspicious manny and so saves the Doctor and Ray by mistaik. We never find out who he was, just that he was a bounty hunter. Maybe he got lost while looking for Han Solo?
Delta decides to trust Mel and Billy and tell them what's going on, but not us viewers because for us it cuts to the next scene and the next day, where the Americans have met a mysterious Welsh manny who tells them about butterflies.
At first it seems this has nothing to do with the main story (like the rest of the Americans' plotline), but it turns out this is a metaphor for Delta when we cut back to her finishing telling her story. This is a much more effective and novel way of conveying the situation than if it had been a pure dump of the exposition, which I'm sure is how Terry Nation would have done it!
When the Doctor and Ray wake up from their sleeps, they tell Mr Burton that the Bannermannys are coming but he doesn't believe them and makes a joke and a skeptical face. Seeing inside the TARDIS convinces him the Doctor is telling the truth. Mr Burton asks to come with the Doctor in the TARDIS - he would have made a great Companion but sadly it is not to be.
Like Billy, and Ray, Mr Burton now takes everything in his stride, with only one scene of self-doubt that is a nice touch. He says
"I am still not sure what I saw in that police box, but I cannot risk my staff for it."and they start to evacuate all the mannys from the holiday camp.
While looking for Billy and Delta, the Doctor and Ray finally meet the Americans but they soon bounce off them which means the Americans still don't join in the main plot.
The Americans then see Gavrok's spaceship land and they think it is the satellite they are looking for because they are silly. After this there is a change of tone - things get more serious now that the Bannermannys are on the scene. Gavrok captures the Americans but, luckily for them, they don't know anything at all about the plot that Gavrok is interested in so they cannot tell him anything useful. Phew!
Things get even more serious when the Bannermannys blow up the spacebus as it tries to escape from them, and Gavrok captures Mel. Mel tries to be brave and lies about Delta being ded in the hope that it will make Gavrok go away - it nearly works except that Delta arrives right then and there. Oops.
Mr Burton is also brave and allows himself to be captured as well so he can persuade Gavrok not to kill Mel, so they are both prisoners instead. Prisoners? At a holiday site in Wales? That gives me an idea for a TV series, except I think it has been done already...
The Doctor goes to confront Gavrok under a flag of truce. He interrupts Gavrok while he is having noms, which may make him even grumpier than he normally is. In what is easily Sylvester McCoy's best scene as the Doctor up to this point, he brazens it out with Gavrok and tries to bluff him into releasing his prisoners.
It almost works, and the Doctor, Mr Burton and Mel start to walk away from Gavrok. But at the end the Bannermannys point their pewpewpew guns at them for a cliffhanger!
This is a great episode that starts to pay off some of what the first part was setting up, but it is still tonally uneven in mixing the comedy with the serious bits - even the magnificent dramatic cliffhanger scene has a misjudged moment of comedy as the Doctor, Mr Burton and Mel do a synchronised double-take look back at the Bannermannys, somewhat undermining the carefully built up tension.
Despite that drawback it is still definitely a step up from part one, with the arrival of Gavrok and the Bannermannys on Earth making for a significant escalation of peril, so we have some properly rising action heading in to the final episode.