In 1998, Miles Woodman is a wisecracking African-American 6th grade boy who is an avid fan of sports, mainly baseball icon Hank Aaron, but is failing at school. His teacher Mrs. Clark warns Miles he will have to repeat the year should his grades not improve. He and his class visit a museum, dedicated to Martin Luther King, Jr. Miles and his Caucasian best friend Randy Smith explore Martin's bedroom, and are caught by the museum's curator Mrs. Peck, who winds up an old watch. The boys hold Martin's baseball glove and the two are transported back to 1941 and meeting a 12-year-old Martin playing with his friends, Sam and Skip Dale, until their mother arrives and reprimands her sons for integrating with the "coloreds." Martin explicates to Miles and Randy that Mrs. Dale's hatred of black people stems from the fact she regards them as "different," but violence would only worsen things. In Atlanta, Georgia, where Martin lived, and elsewhere in the United States, there are "White Only" signs. We are told that black people were brought in as slaves long ago and despite being set free some societies still did not treat them fairly.
The boys then travel 3 years in time and meet a 15-year-old Martin on a segregated train heading for home to enter college. He explains to them that blacks and whites are unable to integrate and must use separate public bathrooms, water fountains, lunch counters, schools, and waiting rooms. On the other hand, Martin studied of some black Americans who were honorable, so Martin believes he too could become a leader. They later have dinner with Martin's family and while he goes to do shut in rounds with his father, Miles and Randy travel onward January 30, 1956, and meet Martin, who by now is 26 and works as a minister at a church in Montgomery, Alabama. He is holding a meeting about the Bus Boycott, set off after Rosa Parks, a black seamstress refused to give up her seat on a bus to a white person. As a result, it ignited a movement where no black adults or children will ride the buses, putting the transportation line in jeopardy if desegregation is press for. Just then, Martin's friend Turner alerts him his house has been bombed. He races home, where his wife Coretta King and newborn daughter Yolanda have escaped unharmed.
Turner declares that in retaliation, they will attack the offenders with bricks and Molotov cocktails, but Martin stops him, reminding the crowd of Mahatma Gandhi peacefully standing his ground to exile the British colonies from India and of Jesus teaching love for his enemies. Martin said to his followers "We must meet hate with love." Miles and Randy travel to the Birmingham riot of 1963, witnessing police commissioner Eugene "Bull" Connor leading firemen and officers into squirting black protesters, including kids and some whites, with water hoses and releasing German Shepherds on them. Connor continued issuing the arrest of protesters, but thankfully the incident is put on television. Martin was put in prison for days till Coretta telephoned the White House and had President John F. Kennedy give Martin his legal rights and freedom. Kennedy fired Connor, and the riot effectively made the whole country realize that the laws were wrong and had to change. The boys are transported back to the museum and join their class back at school. The following day, Miles and Randy speak about the events prior to Martin's work and later the class watches a VHS tape of Martin's life, astonish, and amaze of their knowledge. After the class leaves, Maria Ramirez, an intelligence Latino girl, and Kyle Langon another Caucasian who bullies Miles, decide to investigate for themselves how Miles and Randy got the information. When the boys arrive at the museum, Mrs. Peck lets them stay but warns them that when one messes with the past, this can affect the present.
Maria and Kyle follow the two boys in and catch them in Martin's bedroom. The four children are then transported to the March on Washington Movement and meet Martin who is 34. They sat around listening on Martin delivering his "I Have a Dream" speech. A young Miss Clark aspires to become a teacher. Miles and Kyle get along at last. After this, Kennedy would then propose the equality to all Americans, Martin won a Nobel Peace Prize, and the "White Only" signs were abolished for good. When the kids return, they learn Martin was assassinated. The children travel back to 1941 and bring the 12-year-old Martin to the present. When they return, only Miles and Martin were together and the present is different. They discover that the museum is now just a burned down house. They find out Randy and Kyle became racists, and no longer friends with Miles or knows him. His school is segregated and named after Robert E. Lee; Miss Clark is treated poorly by the principal, and Maria works as a maid and can't speak English. Miles and his mother live in poverty.
The next day, Miles still can't understand what is going on but Martin figures out because he left his own time it created an alternate timeline where his civil rights never happened. Martin teaches Miles not to be afraid to make a difference, and Miles must sacrifice his plans of rescuing Martin. Miles bids a tearful farewell, but realizes he still has Martin's watch. Martin returns to his time, to when he is shot by James Earl Ray at the Lorraine Motel in Memphis, Tennessee, on April 4, 1968. This result in the present reverting to normal and Miles is reunited with Randy, Maria, and Kyle. Mrs. Peck knew about Miles' time-traveling and tells him that while they can't change the past as long as they remember Martin and what he stood for he will always be with them. Miles gives back the watch to Mrs. Peck. Miles receives an A+ on his history test, allowing him to progress to 7th grade. He and his friends then vow to continue Martin's work.