It is Old Mr. Grace's birthday, and the department want to bring a musical tribute to the Grace family. Mr. Rumbold's brother Mycroft (played by Nicholas Smith in a double role) then sets out to trace the Grace family tree, which leads to Wales, Scotland, Somerset and, ultimately (apparently), deepest Africa. The title of the episode is a nod to the 1977 American TV mini-series Roots, based on Alex Haley's book of the same title, which aired in Britain shortly afterward. Last appearance of Young Mr. Grace, Old Mr. Grace and Mr. Klein (thetvdb.com)