The plot revolves around a rash of seemingly inexplicable, explosive spontaneous human combustions and Danny O'Neill (Brosnan), a bomb disposal expert that gets involved and will eventually have to solve the case.
The home video trailers marketed it towards the same crowd as Patriot Games and the original Die Hard and the film actually pre-dates and resembles Speed, minus the water portion of the bomb unleashing.
When a Senator is killed in an explosion, the FBI investigates. The agent in charge is Danny O'Neill (Pierce Brosnan), who is separated from his wife Terry (Lisa Eilbacher) (due to the accidental drowning of their only child in their pool) and behaving very erratically. Initially the investigation does not reveal the kind of explosive used or even what was used to detonate it. Eventually it is learned that terrorists have developed an "invisible" liquid explosive which is activated within the human body (by stomach acid). It also does not help that they have to report to Senator Traveres (Ron Silver), the man whom O'Neill's wife is having an affair with and whom Danny also assaulted.
Later, another senator is killed while riding in a limousine; the limo being driven by one of the main villain's henchmen. This henchman is subsequently struck by a moving car, taken into custody and brought into court. Since this henchman is now considered a risk by the main villain, Mikhail Rashid (Ben Cross), the judge in the case is slipped the liquid and she spontaneously explodes, killing the arraigned henchman in the process. It is at this time that O'Neill discovers the cause of the explosions - the chemically enhanced water in the judge's pitcher and later fights the main explosive courier, Al-red (Tony Plana).
It becomes obvious that the next target is Senator Traveres, so O'Neill, concerned that his wife may become collateral damage, trails his every move. He eventually infiltrates the senator's heavily guarded mansion, at a very convenient time as it is being overrun by the bad guys. O'Neill concocts a cornucopia of interesting home-made weapons, even building bombs using fertilizer found in the kitchen cabinet. All the bad guys die, except for Rashid, who holds O'Neill's wife hostage in front of him and Traveres.
The bad guy swallows some of the liquid, conceding his fate, but intending to bring them all down with him. O'Neill manages to free his wife and send her to safe ground. He and Traveres however have nowhere to go and are thus subsequently forced to jump from the third floor due to the explosion caused by the villain exploding. Traveres lands on a wrought-iron fence which impales and kills him. O'Neill is seen dangling by his arm from Traveres. The ending features the hero (O'Neill) bagging the ultimate prize (his wife), having a second child and (presumably) living happily ever after.