Synopsis: The TARDIS interior is unexpectedly transformed into a forest. It continues to subtly alter, as Ace tries to help the Doctor figure out what's going wrong. He knows it has something to do with something he lost in his second life, but can't quite remember what. When a passing raven—one of the new inhabitants of the en-forested control room, gives a ring to Ace, the Doctor's memory is finally triggered.
The ring, which the FIrst Doctor used to wear, and which slipped off the Second Doctor's finger immediately after the regeneration, must've fallen into the TARDIS controls. There, it linked up with the malfunctioning chameleon circuit, where it caused untold damage over the centuries. Only now has it started to manifest itself in ways that the Doctor could overtly perceive.
When the Doctor retrieves the ring and decides to start wearing it again, the interior of the TARDIS is returned to normal.