If this plane were hijacked, what on earth would you do?
Literal
By-chance this airplane [subject-が] was-hijacked-[conditional-たら], you [topic-は] really how do-[deliberative-だろうか]?
乗っ取る ('to hijack, to take over') — the passive 乗っ取られる describes the plane being seized. 一体 ('on earth, really,' formal variant of いったい) adds rhetorical urgency. だろうか ends with a musing, uncertain question.