JustTin is an eight-foot high robot with many remarkable powers, including the ability to be extremely rude to people. He has been built by Dad Trotter with the help of his clumsy ten-year-old son, Barry, and Barry's best friend, Zed.

When he's not tinkering with JustTin in his garage, Dad works as a special government agent for MI3 [?] who are fighting against a sinister terrorist organisation. One summer holiday, Barry, Zed and JustTin are sent to stay with Barry's eccentric Aunt B, who is very badly dressed and a terrible cook but also a renowned government agent. B's house comes under attack from the terrorists, but using JustTin's powers they fight them off.

After this showdown, Dad Trotter goes missing in a plane crash but...