Sick Note is the story of Daniel Glass, a loveable, lazy, loser who regularly fakes illness to get time off work. He's about to get dumped by his long-suffering girlfriend Becca and lose his home in the process. He's also due to get fired from his job at We Cover Insurance, as the CEO Kenny West wants to set an example by sacking useless employees (which Daniel most certainly is).
A trip to receive scan results on a faked arm injury leads Daniel to the office of Dr Glennis, who tells Daniel the terrible news: he has cancer. Daniel is in shock. But once he's done reeling, he starts to notice the good things that happen to him as a result of the diagnosis. Becca wants to be there for him so he's not getting kicked out. His line manager Michael gives him his job back with a pay rise (against Kenny West's profane wishes) and tells him he can take all the time off he needs. Even his parents - usually thoroughly disinterested in their wayward son's life - are suddenly showering Daniel with love and affection.
However, Daniel does not have a terminal illness. Dr Glennis, it turns out, is a very incompetent doctor and Daniel's misdiagnosis might just be the final straw to end his career. Fearful of losing his job and high-maintenance wife, Annette, Dr Glennis convinces Daniel to keep up the ruse and keep his new life. Daniel begins to feel guilty, but as his single lie spins into a web of deceit things have soon gone way too far to come clean.
As Daniel and Dr Glennis attempt to fake the symptoms of Daniel's illness, Kenny West decides to exploit the situation and make Daniel the public face of We Cover Insurance, thrusting him into the limelight and forcing the lie out into the public sphere. All the while, luckless Daniel and his oafish doctor find themselves dealing with the aftermath of a terrible accident when Daniel's best friend Ash overhears Daniel's secret.
To make matters worse, the ambitious Officer Hayward is on their case, convinced that a conspiracy is afoot, and Daniel has to act like everything's normal while his whole world crumbles around him. Even the affectionate attentions of his favourite work colleague Linda can't take Daniel's mind off the fact that he could be found out any day.
For someone who only wanted an easy life, Daniel Glass has found himself in a world of trouble. He has made a terrible decision, and the consequences are going to prove to be almost impossible to live with.