During the Chimeran invasion of the United Kingdom, Seaver was stationed in Manchester, sometime in late 1950. As Chimeran forces began laying siege to Manchester, Seaver, alongside his fellow medic Corporal Lewis were posted in the Manchester Cathedral where they did their best to treat wounded British soldiers. During this period, the Cathedral served as a makeshift hospital with plenty of supplies stocked in it.
As Chimeran forces began laying siege to British stronghold in the Cathedral, they deployed Spire Missiles, which in turn deployed Crawlers. These attacks often sapped British manpower, however, Seaver and Lewis began realizing that other still active Crawlers would not attack infected bodies. Unable to treat the infected men, both medics issued orders to the remaining soldiers to pile their comatose comrades onto the Cathedral walls, in an effort to deter the remaining Crawlers from entering the stronghold.
One night while on patrol, Seaver, Lewis and a fellow medic were attacked by Crawlers, fortunately Lewis and the other medic were able to escape the Crawlers, however, Seaver was not so lucky and was infected by the Crawlers. Out of desperation, Lewis and his fellow medic decided to utilize morphine to stop Seaver's heart, before extracting the Crawlers out of his body. Strangely however, both Lewis and the medic discovered that the Crawlers inside were already dead.
It was left unclear if Seaver had died shortly after the morphine injection, or if he had merely been rendered comatose following the infection.