The Blue Tooth

"I suppose that was one of the Doctor's most endearing qualities: the ability to make the bizarre and the terrifying seem utterly normal."

When Liz Shaw's friend Jean goes missing, the Doctor and U.N.I.T. are drawn to the scene to investigate. Soon Liz discovers a potential alien invasion that will have far-reaching effects on her life... and the Doctor is unexpectedly re-united with an old enemy…