Adam was a genius, having successfully hacked into the United States Department of Defense computers when he was eight years old, nearly causing, in his own words, World War III. He was recruited by Van Statten. The Doctor and Rose Tyler first met Adam in his early twenties. He worked at the time in Van Statten's Vault in Utah where Adam cataloged extraterrestrial artefacts for his employer. When the Dalek Van Statten had "collected" got loose and went on a rampage, Adam found himself running from it along with Rose. At the end of this adventure, Rose asked the Doctor to take Adam along with them in the Doctor's TARDIS as Adam had told her earlier that he had always wanted to see the stars. Rose also found Adam attractive. Despite the Doctor's scepticism about Adam as a potential fellow-traveller, he agreed (DW: Dalek).
Adam's travels with the Doctor and Rose would not last long. Visiting the year 200,000, Adam was overwhelmed by the wealth of information and technology available to him and ultimately gave in to temptation. He had a top-of-the-line computer interface port known as an infospike installed in his head. Unfortunately for 21st century sensibilities, when the implant activated, his head opened up, revealing part of his brain. The implant activated by a snap of the fingers so he could access the future's computer systems and attempted to transmit information back to 21st century Earth using Rose's "Superphone". (Along with the interface port he also got the Vomatomatic for free.)
When he discovered this breach of his trust, the Doctor angrily returned Adam to his home despite much apologetic pleading and destroyed the answering machine which had received the information. The Doctor observed that Adam would have to live a quiet life from now on, lest someone discover the implant in his head and dissect him to find out where it came from. (DW: The Long Game)
Adam is the only known companion to be actually expelled from the TARDIS crew due to bad behaviour. yeah hes very dumb
Reference on website
In 2005, the website "Who is Doctor Who?" announced that 14 year old Adam Mitchell had won a competition arranged by Van Statten the previous week. Adam's winning essay on "Why I Want To Meet An Alien" focuses on acquiring advanced knowledge from them ("I don't think it's cheating, really. It's just a shortcut"), foreshadowing both his work for Van Statten and his actions later.
Behind the Scenes
Disregarding one-off companions, Adam Mitchell is the only companion from the revived series not included in the Journey's End the finale to Series 4 of Doctor Who. Adam is also the only companion of the Ninth Doctor to not also be a companion of the Tenth Doctor.