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On the 18 of February 2011, a Palestinian engineer by the name of Dirar Abu Sisi boarded a train in eastern Ukraine. He was travelling to Kiev, where he hoped to apply for Ukrainian citizenship. But when the train arrived at its destination the following morning, Mr Abu Sisi was no longer on board - he had vanished.

For more than a week, nothing was heard from Mr Abu Sisi, a manager at Gaza's main power plant - then his wife got a phone call, her husband was in an Israeli jail. Now he is awaiting trial, accused of being the brains behind Hamas' rocket programme.

Only twice in the country's history has Israel abducted someone on foreign soil to bring them back to face trial at home. Adolf Eichmann, one of the principal organizers of the Holocaust, was kidnapped in Argentina in 1960, and subsequently tried and executed. In 1986, Mordechai Vanunu was drugged and smuggled out of Italy after revealing the existence of Israel's nuclear programme.

So who is Dirar Abu Sisi? Did he really study rocket science at a Ukrainian military academy, as the Israeli indictment claims? Is he a senior Hamas operative? Or is he an innocent victim of mistaken identity? What role if any did the Ukrainian authorities play in his disappearance from that train?

In this edition of Assignment, Gabriel Gatehouse unravels the mystery of Dirar Abu Sisi, tracking his journey across Ukraine and beyond, to Israel and Gaza. It's a story that involves the secret services of at least two nations, and goes to the very heart of the conflict between the Israelis and the Palestinians.