Threshold is a series which is set in the present. Dr. Molly Caffrey is a crisis management consultant in a think tank that works for the government, whose job is to prepare worst case scenarios for any type of contingency. These contingency plans are detailed and thorough, with procedures intended to deal with any situation that could be encountered during one of the plans. Some of the scenarios are considered to be quite unlikely.
A US Navy freighter encounters an alien probe which affects the crew in different ways, and as a result of this, one of Dr. Caffrey’s plans goes into effect, called Threshold, which deals with alien invasion. A new organisation is formed, one which very few actually know the truth about, also called Threshold. The organisation has a lot of staff, but the core is a Red Team consisting of Caffrey, J.T. Baylock, the Deputy National Security Advisor, who heads the Threshold organisation, Dr. Nigel Fenway (played by Star Trek: The Next Generation‘s Brent Spiner), of NASA, Lucas Pegg, a weapons designer, Arthur Ramsey (played by Game of Thrones‘ Peter Dinklage), a mathematician and linguist and Sean Cavennaugh, who provides military leadership, although he doesn’t seem to serve in the military.
Initially, they fly out to the freighter and discover that some of the crew are dead, and have suffered mutations which have killed them. Others are missing. The missing crew are called infectees; they have been affected by a signal given off by the alien probe – as were those who died – and have been changed. They are trying to spread the infection to unaffected humans, and Threshold’s job is to stop them.
The series only ran for one season of thirteen episodes, and the last few episodes were not originally aired; poor ratings following an ill-advised shift in broadcast time led to the series being cancelled. Despite an attempt to provide at least some sort of ending in the final episode, as the series was cancelled during the filming of that episode there were many questions left unanswered and many plot lines that were completely undeveloped after their initial appearance. There doesn’t seem to have been any attempt to tie these up, through other media, although possible future plot developments were mentioned in the DVD commentary.