Martin was possessed by the Ancient Lights. He began to give highly accurate readings at a local theatre, quickly gaining popularity. He started a show in which he demonstrated his talents before a live audience. His act caught the attention of the Psychic Channel, which gave him a medium to enact the Ancient Lights' master plan on a nationwide scale. He also took an interest in Sarah Jane Smith, knowing she had travelled with the Doctor.
The power of the Ancient Lights enabled him to fire energy blasts and exert control over his clients, as well as give accurate information about the past and future.
When his plan of possessing the world and making them "join the circle and the Ancient Lights" was foiled by Luke Smith, he said it was his destiny and disintegrated into space, allowing the Ancient Lights to make him "one with the stars". (TV: Secrets of the Stars)
When he was a con artist, Trueman was a cynical man who did not believe in astrology. He seemed to possess some morals however, as he visibly shocked and ridden with guilt upon learning that Cheryl was paying him with her mortgage.
When he was possessed by the Ancient Lights, Trueman became inspired and embraced his position as the 'Chosen One'. When his plan failed, he refused to abandon the Ancient Lights, unable to bear the idea of returning to his former life, instead choosing to become "one with the stars". (TV: Secrets of the Stars)
The case of 'Trueman the astrologer' was one of several alien incursions which occurred in the 'Ealing Triangle', cited by Ruby White when she met Sarah Jane and her team. (TV: Goodbye, Sarah Jane Smith)