The series revolves around the eponymous Joe, a nine-year-old schoolboy and the adopted son of widowed computer expert Professor Ian "Mac" McClaine. Ostensibly an ordinary father-and-son pair, the McClaines live in an Elizabethan-style cottage on the Dorset coast. In the basement of the cottage is a secret laboratory containing Mac's latest invention, the Brain Impulse Record And Transfer (referred to by the acronym "BIG RAT"): a machine capable of recording a person's knowledge and experience and transferring it to the mind of another. The BIG RAT is centred around the "Rat Trap": a spinning, spherical cage in which the pre-recorded "brain patterns" are uploaded to the recipient.