The group of Africans are charged with mutiny and murder. There are also property claims by the Queen Isabella II of Spain, the captain of the ship, the people who captured the Africans, and others. A young lawyer, Baldwin, is brought on the case by the abolitionists.
Baldwin finds a translator (Chiwetel Ejiofor in his first film role) and talks to Cinqué, who has become the leader of the group of Africans. In flashbacks, Cinqué tells about his life.
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He was captured in Africa and brought to the Caribbean Islands by an infamous Middle Passage slave vessel named the Tecora. Cinqué tells of the various horrors of the Middle Passage, such as when fifty people were drowned to save rations. Cinqué was finally taken to the Caribbean Islands, where he was illegally sold to the owners of La Amistad.
In the district court, Baldwin brings as evidence a book he found on the ship. It conclusively proves that the Africans did indeed come from Africa. The defendants’ birthplace was a matter of critical importance because U.S. law at the time outlawed anyone who wasn’t the child of a slave from being enslaved (based on the provision in the United States Constitution that permitted Congress to outlaw the African slave trade starting in the year 1809). This meant that those held aboard La Amistad were being traded illegally, and were officially abducted citizens of West Africa. As such, they were legally permitted to use deadly force to secure their release (thus making the killings upon the ship justifiable homicide rather than murder). Thus whether or not the defendants were born in Africa was critical to determining whether their conduct was indeed justificable homicide or murder.
Upon seeing the log book from the ship, the presiding judge appears prepared to rule for Baldwin. But President Van Buren, under pressure from the South, replaces the judge with a younger judge who Van Buren can influence. It is thought that this will result in a ruling against the defendants. However, the new judge surprises the Administration by ruling in favor of the defendants. The judge concludes that they were born in Africa, and thus were permitted to use deadly force to resist those that would enslave them. He orders their return to Africa at Government expense, and further orders the arrest of the would-be slavetraders.
The prosecution then appeals the case to the Supreme Court, where seven of the nine justices are slave owners. Baldwin finally convinces former president John Quincy Adams to help him on the case. After some communication with Cinqué, Baldwin and Adams are ready to present the case (Baldwin making the case to the Supreme Court isn’t shown). John Quincy Adams then gives a speech on slavery and the case in general. The Supreme Court then (March 9, 1841) rules in favor of the Amistad Africans, in an opinion by Justice Joseph Story. Story was played in the movie by an actual retired Supreme Court justice, Harry Blackmun.
The end of the movie notes that Cinqué returned to Africa, the slave fortress he went through was destroyed by the Royal Navy, and the American Civil War was fought over many issues, slavery among them.
