Why did Sean get onto the plane in the Event?

In the first episode of the great but short-lived NBC TV series "The Event", Sean Walker is shown getting on a plane that is being flown to Coral Gables, Florida to assassinate the President. The plane is being piloted by Sean's girlfriend's father Michael Buchanan, who is being forced to participate in this assassination attempt because his family is being held hostage. Sean is shown trying to get into the cockpit and stop Michael from crashing the plane.

But my question is, why did Sean get onto the plane? As far as I can tell, chronologically the last time we saw Sean before thist was in a flashback where he had found out that his girlfriend Leila Buchanan was missing in her hotel room. He asked the front desk where she was, and the hotel had no record of Sean or Leila, the cruise they were traveling on had no record of them either, and Leila's phone number had been disconnected. Hotel security tried to apprehend him, because they thought he had snuck onto the cruise or something, but he ran away before they could catch him.

But what happened between that scene and him being on the plane? They say (in a later episode) that he used a false ID as a flight crewman or something to get on the plane, but what was his motivation for doing this, how did he know the flight was being used for an assassination attempt, and how did he know that his girlfriend's father was piloting it? How did he go from looking for his girlfriend to trying to save his girlfriend's father and/or prevent presidential assassinations? And then afterwards, of course, he goes back to looking for his girlfriend, so the whole going on the plane part seems out of place.

Can anyone shed light on this?