The claims are part of an elaborate Mugabe-inspired hoax. Zanu-PF has tried on numerous occasions to claim that Tsvangirai is trying to assassinate Mugabe. That way, they can use anti-terrorist laws against him - or get their death squads to lynch him. The president of the Canadian consultancy that 'exposed' the plot is Ari Ben Menashe, a former Mossad agent formerly involved in the Iran-Contra arms scandal and allegedly played a part in the assassination of Robert Maxwell. He has a long association with Mugabe. Naturally, he is not thought of as a trustworthy man, even by his former Israeli employers. He is a mercenary prepared to carry out underhand tactics for whoever pays the most.

How could this be a hoax, when they have video footage of Tsvangirai discussing the plan to assasinate Mugabe?

Its a hoax because it shows an edited video tape.
the head of the opposition party says
"When Mugabe has been removed" tape jumps then conversation continues.
The argument is that by removed they mean assinated - but it could have been "Removed from political power" or any other form of words - hardly the worlds most devious plot