Based on the 1970s The Dandy comic adventure strip Peter's Pocket Grandpa by Ron Spencer (which itself was based on an earlier prose text story called Jimmie's Pocket Grandpa which had appeared in The Dandy in the 1940s), the programme stars James Bolam as a grandfather who owns a magical "shrinking cap", which only his 10-year-old grandson Jason Mason (played by Jay Ruckley) knows about.

This cap enables him to shrink to about 4 inches tall, become a CGI-animated character, run very fast, and bring toys and objects to life.

According to the BBC press office, "Grandpa in My Pocket explores the hugely important relationship between grandchildren and grandparents by turning it on its head".

Jason and Jemima's younger cousins Josh and Elsie (played by Harvey Thorn and Sezen Djouma) have come to stay for the summer, and the two become the series narrators as Grandpa decides to let them in on the secret of the shrinking cap.

In the United Kingdom, the first season was released on DVD by Warner Home Video from 2010 to 2011, with Decode Enterprises securing the deal in July 2009.