Tikka to ride is that rarest of things, a good story about a real life tragedy.
The Dwarfers travel back in time in search of a curry (This is Lister we are talking about) and end up in the Texas Book Depository on November 22nd 1963. They alter the course of events so that Kennedy does not die, saving a popular and charismatic president.
Then they try and return to Starbug, only it is not there anymore. You see Kennedy was Impeached shortly into his presidency and his vice president was blackmailed by the maffia and ultimately the US lost the Cold War.
They go back to try and fix the mess they go back and re-direct Lee Harvey Oswald to a higher floor, unfortunately Oswald now only injured Kennedy due to the increased difficulty of the shot. Now the only way to be certain that Kennedy dies is to get a second gunman and the only place for such a gunman is the Grassy Knoll. Now the problem is, who to shoot?
This is when the episode really works. They go into the future and get the one person who has the moral authority to shoot JFK: John F Kennedy himself.
Beautifully done and treated with the respect it deserves while still being funny. The extended version is even better and well worth a look if you can find it, it ties up some of the plot holes the main episode has (not that is ever a problem for Red Dwarf
now is it?) and expands on some of the funnier scenes.