Major Shapp was an aide to the Marshal of Atrios.
While investigating an intrusion, Shapp travelled through a hidden transmat to Zeos. He assisted the Fourth Doctor in searching the planet. They discovered a computer called Mentalis which was controlling the Zeos war effort. Shapp tried to destroy it but the computer's automated defence mechanism disarmed him.
The Doctor instructed Shapp to return to Atrios. As he prepared to transmat, a Mute attacked him. Shapp killed him but was himself injured; falling into the transmat, he returned to Atrios. He contacted the Marshal and told him to call off his attack on Zeos, but was ignored. After the defeat of the Shadow, the war ended. (TV: The Armageddon Factor)