According to Comic Basics' ending explainer, the CIA's deputy director Mosier believes Sean stole a nuclear warhead and suspects his friends, which is why they wake up bound on a plane after drinking spiked shots. The real culprit is Helen, Sean's handler (the character is called Ace in other reviews), played by Danai Gurira. She arranged the theft out of ambition, to frame Sean and his friends and to detonate the weapon on friendly soil, provoke a war and then be the hero.
The trail leads to Bucharest and an arms broker named Robbie, who modified the warhead with a proximity failsafe, so the group cannot separate from it. In the helicopter showdown, Sean jumps out with the detonator while Helen is tied up inside; her helicopter crashes and kills her. Sean uses the detonator to safely disarm the bomb, and Mosier's team takes it into custody, a clean resolution. The film ends with Sean and a Camaro, completing the childhood car motif, and with him reconnected to his friends.