Dark Sun is a Cherub book created specifically for World Book Day 2008.
Rat is going undercover in Milton Keynes to befriend George Lydon, son of nuclear engineer and suspected member of nuclear trafficking network "Dark Sun" Kurt Lydon. The mission is going well, and Rat has managed to get himself and Andy Lagan, who is posing as Rat's cousin, invited to George's summer sleepover. On the last day of term, George's other friend Zhang finds a rancid tub of coleslaw in his locker and convinces George to throw it out a window onto George's sister Sophie. Unfortunately, the coleslaw hits a rugby player called Thomas Moran, and Thomas runs after the trio. Rat manages to knock out Thomas, strengthing he friendship between him and George.
Back at campus, Lauren, Andy, Jake and several of Jake's mates are punished for mucking around during a lecture by doing the assault course. Andy is knocked off the height challenge and falls into the pond, seriously pulling a muscle in his left upper body, but pushes through the pain barrier to go on the mission.
Andy and Rat arrive at George's house and settle down for an Xbox tournament. Sophie goes out clubbing and George's mother Dr. Lydon goes to bed early after a long day at the hospital. With Kurt away at a meeting in Belgium, all Rat and Andy have to do is knock out George and Zhang with some beer spiked with sedative, spray knock-out gas into Dr. Lydon's room and change Lydon's designs for a uranium centifruge. However, Sophie comes home early and, as her door is locked, goes to Dr. Lydon's room to ask her mum to unlock her door. Rat distracts Sophie while Andy opens Sophie's room, but Sophie is freaked out and smashes a vase over Rat's head. Andy manages to inject Sophie with a sedative, and the two cherubs finish their work.
With the mission's objectie completed, Rat and Andy return to campus. A new, state-of-the-art library is opened on campus on World Book Day. James and Lauren join Rat and Andy at the opening, and Rat shows Andy the newspaper headlines: Kurt Lydon and twenty-one other members of Dark Sun have been arrested after the faulty centifruge explodes in a Nigerian uranium enrichment facility.