Synopsis: The Doctor and Kul continue their descent to the planet's core. While the Doctor is trying to determine if the gangly threads they've been rappelling on are hairs, nerves or branches, the duo is beset by some blob like creatures. Eventually he works out that the "threads" are indeed nerves and the "blobs" are antibodies.
The planet is alive.
After finding the TARDIS enmeshed in a web of these nerves, the Doctor himself is soon hardwired into the planet's brain. The Doctor receives a message through the nervous system. The planet reveals its secrets.
Like the Time Lord, the planet is a lonely creature. It has drawn spaceships to its surface merely in order to have someone to talk with. Because it has no way of controlling its own antibodies—which naturally attack organic matter as disease—the planet has reached out to the computer systems of all the vessels that have landed on its surface. It has drained the contents of each one, including the TARDIS. The Doctor's vessel, however, it recognizes as singular: "It sings. It has . . . a soul."
The Doctor proposes that with Kul's people now stranded on the surface, the planet has a people who are quite willing to live in a symbiotic relationship with it. However, he still has to find a way to help the planet control its threatening antibodies. Using his sonic screwdriver, the doctor performs "a little surgery to the nerve-root ganglia". This has the effect of reprogramming the antibodies to not view Kul's people as threats.
Noting blossoms now growing on the planet's nerve tendrils, the Doctor suggests that "everything will be coming up roses" for Kul and the planet.
The planet releases the TARDIS and the Doctor disappears, saying, "I've got my own Roses to think about."