Window cleaner Tim Bradshaw, a naive twenty-two year old Londoner, is about to join London Underground in search of a better working life. After cheating on his aptitude test, he is offered employment as a Station Assistant. During the recruitment process, he meets another recruit named Richard, and they become the best of friends.
Tim's station training is marked by a series of hilarious pitfalls, as time and again his naivety, and incompetence, lead him into difficulties - even landing him in the hospital a few times. Despite his growing reputation around the network for cock-ups, Tim finds romance with fellow station assistant Carol Wright, and eventually moves into her flat, although he faces competition from fluffy cat Cuddles.
Encouraged by Carol, Tim decides to seek promotion to train driver and to his shock is accepted, although his training continues to be marked by mishaps and mistakes. Tim and Carol take holiday in Greece and Tim proposes, to Carol's delight. However, on their return, a prank at work sends him packing to his parent's home. Fortunately the truth is revealed and they eventually make it to the alter.
Returning from their honeymoon, they discover the union has called a strike, leaving Carol in a panic over finances. Carol ends up working for Tim's dad at the local chicken farm, and although Tim holds out as long as he can, he ends up cleaning windows until the strike is finished, if only to show Carol how much he cares. When Carol discovers she is pregnant, the couple look for a larger home near Tim's parents. Carol gives birth to a baby boy called Tim, and they live happily ever after.