Spoiler warning: Spoilers for the complete film.
Nick survived and betrayed the Union, working with Juliet to sell its intelligence. His apparent death concealed a change of allegiance. The Union’s final mission is therefore a fight against someone who knows the organization from inside.
Why does Nick turn against the team?
He wants the money and independence the sale can provide, rather than remaining another operative doing dangerous work for the agency. Juliet’s auction creates a market for information he was supposed to protect.
His return does not restore the partnership Roxanne lost. It forces her to recognize that shared service is not proof of shared priorities.
That reversal also explains why the stolen material is so dangerous. Knowledge gathered for an institution can be used against its people once someone treats it as a personal asset.
After headquarters is attacked, they rely on help from one of Mike’s friends to get out of the country and intercept Nick and Juliet. They recover the intelligence and stop it reaching the intended buyers.
Mike’s established life becomes useful rather than merely embarrassing background he must escape. The same ordinary connections that distinguish him from a polished spy help keep the mission moving when institutional resources fail.
The film’s blue-collar premise is clearest at that point. Recruiting an outsider has value beyond giving Roxanne someone who has not yet been exposed inside the organization.
Does the mission resolve their relationship?
Working together gives Mike and Roxanne a new relationship to consider, built on action in the present rather than only their history as teenage sweethearts. It does not make the intervening years disappear.
Nick and Mike offer different responses to belonging. Nick decides his knowledge should buy him a separate future; Mike risks the comfortable life he already has to help someone else.
The successful recovery closes the immediate threat to the Union. The personal consequence is that Roxanne can no longer think of Mike simply as the man she left behind, or of Nick as the colleague whose loyalty she could take for granted.
Sources: official ending breakdown.