The title is a reference to the biblical sentence "And whosoever shall compel thee to go a mile, go with him twain" as in St Matthew, Chapter Five, Verse Forty-One.

The third mile could also indirectly refer to a particularly elaborate scheme used in the book to lure three of the college staff to London.

During the Second World War, there are three brothers Albert, Alfred and John Gilbert serving as tank drivers in the army.

A few days later, the Master of Lonsdale College invites Morse to lunch and voices suspicion about the disappearance of Browne-Smith.

As more evidence accumulates, suspicion seems to fall on the Don Westerby who was antagonistic towards Browne-Smith at Lonsdale College and is supposedly on a holiday in Greece after his recent retirement.

During a visit to Browne-Smith's room at the college, Morse meets one of the Gilbert twins who is running a packers and movers business engaged to shift the retired Westerby's effects.

In a moment of forgivable lapse brought about by his fear of corpses, Morse does not recognise the murderer and allows him to walk free.

The second mile concludes with three more deaths - one murder, one suicide and one from natural causes, leaving none of the suspects alive.

The episode guest stars Barry Foster as Sir Alexander Reece, Michael Aldridge as Arthur Drysdale and Tenniel Evans as Doctor David Kerridge, three characters who combine aspects of Browne-Smith, Westerby, the Gilbert twins and the Master.