Harry wakes up two more mannys - Lycett and Rogin, both of whom have proper space-sounding names - to help them. The Doctor has a plan to beat the Wirrn when they have a sleep. He starts his plan by plugging his brain into the dead Wirrn's brain so he can see what killed it. The Doctor is not Mr Spock yet so he has to use electrics to do this. Everyone else says this is very dangerous but the Doctor laughs and does it anyway.
The green thing wriggles in very fast and kills Lycett, trapping the others in the room where the Doctor is seeing inside the Wirrn's brain. Harry and Rogin run to get guns. They meet Noah (who is even more green now) and shoot him, then they run in and - directed by the Doctor - shoot the other green thing in its nipples.
The guns don't kill the aliens but shooting them in the nipples does make them go away. This shows that the Doctor has learned something about their weakness from the dead Wirrn's brain.
To escape from the room they use the teleport, but Noah turns the power off so they are separated and will be running out of air soon. This is not a dramatic enough turn of events to end the episode on, so the Doctor goes to see if it is safe and he sees Noah finally turning all the way into a Wirrn.
In some ways not a lot happens in this episode, but it is full of suspense and alien green things, so the Doctor and his friends are always in peril. This builds up until the maximum moment of danger at the end of the episode, which will have to be resolved next time because it is the last part of the story.